Installer refactoring

WIP
@daniel-nichter I'm still not getting this flag "create-mysql-user". It looks like that even we add "-create-mysql-user=false" then it still asks for MySQL user credentials:
```sh
"CTRL-C at any time to quit\n"
"API host: http://127.0.0.1:55689\n"
"API key: "
"Verifying API key 00000000000000000000000000000001...\n"
"API key 00000000000000000000000000000001 is OK\n"
"Created server instance: hostname=localhost id=10\n"
"Create MySQL user for agent? ('N' to use existing user) (Y): "
"Specify a root/super MySQL user to create a user for the agent\n"
"MySQL username: "
"MySQL password: "
"MySQL host[:port] or socket file (localhost): "
"Testing MySQL connection root:...@unix(/var/run/mysqld/mysqld.sock)...\n"
"MySQL connection OK\n"
"Skip creating MySQL user (-create-mysql-user=false)\n"
"Agent MySQL user: root@unix(/var/run/mysqld/mysqld.sock)/?parseTime=true\n"
"Created MySQL instance: dsn= hostname=localhost id=10\n"
"Created agent: uuid=0001\n"
"Install successful\n"
```
Though it skips actually creating it. But why it asks then for super user credentials and also verifies it?