MAR-149: Switch to using File.content_string instead of File.content in tests for all roles:

- The .content variant returns contents as bytes, while the
content_string returns a string (which is what is wanted in
practically all cases).

MAR-148: Fixed tests for the backup_server role:

- Fix some handling of newlines by updating test data keys to include
newline at the end of file.
- Ensure we do not produce exit code 255 when testing the available
authentication mechanisms for the backup SSH server
instance (otherwise we get runtime exception from Testinfra itself).
- Fix some handling of newlines by not doing stripping and including
newline in expected output.

MAR-132: Added support for Debian 9 (Stretch) to backup_server role:

- Deploy slightly different sshd_config file for Stretch (DSA key
unused).
- Updated Molecule tests to cover testing on Debian 9.
- Updated role reference documentation.

MAR-129: Updated backup_server role tests and fixed some issues:

- Updated how the tests are referencing the relevant hosts to run the
tests on.
- Use "state: present" for package installation.

MAR-128: Fixed test for login mechanisms on backup_server role:

- Moved test back into test_default.py to be run against all servers.
- Switch to using local ssh command that would reveal available
mechanisms instead of using paramiko from the test host.

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).