MAR-129: Updated backup role for new Ansible and Molecule:

- Fixed linting errors.
- Include top-level linting configuration in tests.
- Moved variables from test playbook into group_vars.
- Updated how the hosts are referenced in tests.

MAR-128: Upgraded tests for ROLE_NAME 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).