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.