MAR-128: Upgraded tests for backup_server 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).
- Switched to extracting IP address instead of hard-coding it in a
couple of tests.
- Moved test for checking available authentication mechanisms for
backup SSH server to be part of testing of parameters_optional only
for now (it was hard coded to that IP, and fails on
parameters-mandatory due to iptables not opening correct ports).