mysql5.7 の初期にてパスワードがわからなかった時の対応

# 概要
mysql5.7インストール後の初期設定でパスワードがわからないことがあるので、その際の対応を記載する。

# 環境
mysql-server : `Server version: 5.7.26-0ubuntu0.18.04.1 (Ubuntu)`
linux-os : 

```
$ cat /etc/os-release 
NAME="Ubuntu"
VERSION="18.04.1 LTS (Bionic Beaver)"
ID=ubuntu
ID_LIKE=debian
PRETTY_NAME="Ubuntu 18.04.1 LTS"
VERSION_ID="18.04"
HOME_URL="https://www.ubuntu.com/"
SUPPORT_URL="https://help.ubuntu.com/"
BUG_REPORT_URL="https://bugs.launchpad.net/ubuntu/"
PRIVACY_POLICY_URL="https://www.ubuntu.com/legal/terms-and-policies/privacy-policy"
VERSION_CODENAME=bionic
UBUNTU_CODENAME=bionic
```

# 対応
### 初期パスワードを設定する

実行コマンド

```
$ mysql_secure_installation
```

実行ログ

```
$ mysql_secure_installation                                                                                                                                              
                                                                                                                                                                                                          
Securing the MySQL server deployment.                                                                                                                                                                     
                                                                                                                                                                                                          
Enter password for user root:                                                                                                                                                                             
Error: Access denied for user 'root'@'localhost'                                                                                                                                                          
[0:56:08]vagrant@myvagrantbento:~$ sudo mysql_secure_installation                                                                                                                                         
                                                                                                                                                                                                          
Securing the MySQL server deployment.                                                                                                                                                                     
                                                                                                                                                                                                          
Connecting to MySQL using a blank password.                                                                                                                                                               
                                                                                                                                                                                                          
VALIDATE PASSWORD PLUGIN can be used to test passwords                                                                                                                                                    
and improve security. It checks the strength of password                                                                                                                                                  
and allows the users to set only those passwords which are                                                                                                                                                
secure enough. Would you like to setup VALIDATE PASSWORD plugin?                                                                                                                                          
                                                                                                                                                                                                          
Press y|Y for Yes, any other key for No: y                                                                                                                                                                
                                                                                                                                                                                                          
There are three levels of password validation policy:                                                                                                                                                     
                                                                                                                                                                                                          
LOW    Length >= 8                                                                                                                                                                                        
MEDIUM Length >= 8, numeric, mixed case, and special characters                                                                                                                                           
STRONG Length >= 8, numeric, mixed case, special characters and dictionary                  file                                                                                                          
                                                                                                                                                                                                          
Please enter 0 = LOW, 1 = MEDIUM and 2 = STRONG: 0                                                                                                                                                        
Please set the password for root here.                                                                                                                                                                    
                                                                                                                                                                                                          
New password:                                                                                                                                                                                             
                                                                                                                                                                                                          
Re-enter new password:                                                                                                                                                                                    
                                                                                                                                                                                                          
Estimated strength of the password: 25
Do you wish to continue with the password provided?(Press y|Y for Yes, any other key for No) : y                                                                                                          
By default, a MySQL installation has an anonymous user,                                                                                                                                                   
allowing anyone to log into MySQL without having to have                                                                                                                                                  
a user account created for them. This is intended only for                                                                                                                                                
testing, and to make the installation go a bit smoother.                                                                                                                                                  
You should remove them before moving into a production                                                                                                                                                    
environment.                                                                                                                                                                                              
                                                                                                                                                                                                          
Remove anonymous users? (Press y|Y for Yes, any other key for No) : y                                                                                                                                     
Success.                                                                                                                                                                                                  
                                                                                                                                                                                                          
                                                                                                                                                                                                          
Normally, root should only be allowed to connect from                                                                                                                                                     
'localhost'. This ensures that someone cannot guess at                                                                                                                                                    
the root password from the network.                                                                                                                                                                       
                                                                                                                                                                                                          
Disallow root login remotely? (Press y|Y for Yes, any other key for No) : y                                                                                                                               
Success.                                                                                                                                                                                                  
                                                                                                                                                                                                          
By default, MySQL comes with a database named 'test' that                                                                                                                                                 
anyone can access. This is also intended only for testing,                                                                                                                                                
and should be removed before moving into a production                                                                                                                                                     
environment.                                                                                                                                                                                              
                                                                                                                                                                                                          
                                                                                                                                                                                                          
Remove test database and access to it? (Press y|Y for Yes, any other key for No) : y                                                                                                                      
 - Dropping test database...                                                                                                                                                                              
Success.                                                                                                                                                                                                  
                                                                                                                                                                                                          
 - Removing privileges on test database...                                                                                                                                                                
Success.

Reloading the privilege tables will ensure that all changes
made so far will take effect immediately.

Reload privilege tables now? (Press y|Y for Yes, any other key for No) : y
Success.

All done!
```