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.

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.