MAR-129: Updated Molecule configuration for the backup_client role:

- Updates necessary for use with Molecule version 2.14.
- Reference custom yamllint configuration file with custom rules.
- Fix YAML linting errors in Molecule's own files.

MAR-128: Upgraded tests for backup_client 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 SSH deployment to have chroot the users for a
bit easier set-up, and updated relevants playbooks/tests for this
purpose.