MAR-22: Updated tests for common role to work against Debian Stretch:

- Use different package name in test playbook for MariaDB client development
libraries based on debian release.
- Be more specific in task that sets-up compatibility symbolic link for
mysql_config.
- Replaced one of the CA sample certificates to make tests less ambigious (use
different subject DN).
- Removed some unused CA certificates.
- Updated a couple of tests to distinguish between Debian Stretch and Debian
Jessie.

MAR-22: Initial test implementation for role 'common':

- Added Molecule configuration covering all platforms with two hosts for testing
mandatory and optional parameters, as well as helper machine.
- Created test playbook for deploying necessary roles to servers with specific
configuration settings.
- Added sample data used by the test playbook.
- Added stock test file.