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).