Mysql password hash support

Resubmission of #5633 as requested this has been converted into a separate option such that the password_hash may be passed in place of the password itself.

testing via:

```
---
- hosts: all
  tasks:
    - name: mysql_user test password hashed
      mysql_user: user=test password_hash=*973D12C21B4F274E09461F56EEAB92AB7B6FC20A priv=*.*:ALL,GRANT
    - name: mysql_user test2 password not hashed
      mysql_user: user=test2 password=ansible priv=*.*:ALL,GRANT
```

Both users result in having the same password hash.