MAR-192: Added support for Debian 12 Bookworm to backup_server role:

- Use test parametrisation instead of looping over a list when testing
correct key usage.
- Replace deprecated key algorithm (in Debian 12 Bookworm) for test
purposes (it just needs to be one of the RSA variants).

MAR-181: Drop support for Debian 9 Stretch from the backup_server role:

- Switch to using IPs from VirtualBox default allowed host-only
network subnets.

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: Improve the SSH connectivity tests in backup_server role to be more reliable:

- Introduce a session-level fixture for setting permissions for client
SSH private keys (fixes errors related to SSH requesting tighter
permissions).
- Add assertions for the tests that verify the backup clients cannot
connect to the regular SSH server in case the SSH private keys do
not have correct permissions (just in case).

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).