MAR-218: Switch to using fully-qualified collection names for all tasks:

- Ensures there is no ambiguity when invoking a module.

MAR-218: Fix linting errors for unnamed plays:

- All plays should be named at this point.
- Improve naming of some plays.
- Group some of the common tasks together to speed-up the prepare
step.
- Minor refactoring to make the prepare playbooks more consistent
across the roles.

MAR-129: Minor preparation for updating Ansible version:

- Ignore the pytest_cache directory in Git.
- Updated Ansible to latest 2.3.x version in requirements file.
- Make sure the SSH client private keys have correct permissions when
testing the backup_server role (must be 0600), otherwise we get
errors from SSH due to insecure permissions during testing.
- Updated test for testing installed packages in the wsgi_website
Python virtual environment to test against up-to-date
packages. Might be nice to fix this one with something less hacky in
the long run.

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).