MAR-129: Updated backup_server role linting and test configuration:

- Fixed linting issues.
- Use global linting configuration file.
- Moved test variables into group_vars.

MAR-128: Upgraded tests for common role:

- Switch to new Molecule configuration.
- Updated set-up playbook to use become: yes.
- Moved some preparatory steps outside of the main playbook (easies
idempotence tests).
- Updated tests to reference the yml inventory file.
- Updated tests to use new fixture (host instead of individual ones).
- Switched to extracting IP address instead of hard-coding it in a
couple of tests.
- Fixed invalid indentation on task responsible for creating the
pipreqcheck Python virtual environment.