Use explicit credentials if password-less access is not possible

[bajo]

When we disable the management of /root/.my.cnf with percona_server_user_root_cnf_manage: false we lose the ability to run ansible tasks in our playbook.

Adding login_user and login_password let's us use the root password to run the tasks.
check_implicit_admin makes sure to first attempt the password-less login if possible.

[tersmitten]

The only usecase I can think of to set percona_server_user_root_cnf_manage to false is when using percona-client.

[bajo]

Our use-case is that we do not want to have the mysql root user's password stored in /root/.my.cnf. If we set percona_server_user_root_cnf_manage: false, the playbook fails however.
Adding the changes from the PR allows us not to store the password in the file, but still run the playbook.

[bajo]

@tersmitten it might help to clarify our use-case a little better.
We do not want every local sys-admin to be able to read the whole database without a password.
If they need access, they should at least need to retrieve the password from our password-manager, and enter it manually afterwards.