MAR-149: Switch to using File.content_string instead of File.content in tests for all roles:

- The .content variant returns contents as bytes, while the
content_string returns a string (which is what is wanted in
practically all cases).

MAR-129: Updated database role for new Molecule and Ansible versions:

- Fixed linting issues.
- Moved test variables into group_vars.
- Renamed the server which effectively tests the optional
parameters (mainly the backup).
- Updated tests to use correct mechanism for obtaining the list of
hosts on top of which the tests should be run.

MAR-128: Upgraded tests for database role:

- Switch to new Molecule configuration.
- Updated set-up playbook to use become: yes.
- Moved some preparatory steps outside of the main playbook (eases
idempotence tests).
- Updated tests to reference the yml inventory file.
- Updated tests to use new fixture (host instead of individual ones).
- Updated backup_server and backup_client configuration in order to
make the backup testing more generic (and easier to update for
future).