Fix clean install to nonstandard datadir

[gutleib]

Description

I've created a small check if custom datadir exists and is empty. If it is, default files from /var/lib/mysql will be copied there.

Related Issue

This is my fix to the #27, that seems to be fixed for updates, but not for fresh installs.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Chore (non-breaking change that does not add functionality or fix an issue)

Checklist:

• I have read the CONTRIBUTING document.
• I have run the pre-merge tests locally and they pass. - Sorry, I'm having problems with this step.
• I have updated the documentation accordingly. - No need, change is a bugfix
• I have added tests to cover my changes.
• All new and existing tests passed. - Sorry, I'm having problems with this step. ERROR! the role 'ansible-mariadb-galera-cluster' was not found