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: Added missing tests for common role and did small clean-up:

- Removed deployment of SSH client configuration from helper machine.
- Added tests for verifying backup configuration (deployed via meta
dependencies).

MAR-22: Implemented tests for the common role:

- Added missing documentation for pipreqcheck_uid and pipreqcheck_gid
parameters.
- Use static-hashed passwords for reproducibility during testing in test
playbook.
- Install Emacs and libmariadb-client-lgpl-dev-compat via test playbook on one
of the testing instances in order to test related tasks.
- Fixed parameter for connection limitting in test playbook.
- Added explicit parameters to test playbook for pipreqcheck_gid and
pipreqcheck_uid.
- Fixed deployment of ferm configuration file ot include setting user/group and
mode.
- Added tests covering common deployment, deployment when only mandatory
parameters are provided, and deployment when optional parameters are set as
well.