MAR-218: Force handler execution during testing for consistency:

- Otherwise it can easily happen that some handlers never run, leaving
the machine in inconsistent state until they are run by hand.

MAR-218: Drop linting definitions from Molecule configuratin files:

- Molecule no longer natively supports running linters.
- Disable the Ansible Galaxy dependency installation (none of the
roles utilise this).

MAR-189: Disable use of KVM paravirtualisation provider with Vagrant/VirtualBox:

- The VMs seem to randomly hang when the (KVM) clock source gets
initialised right after getting past the bootloader. Helps run the
tests in a way more reliable way.
- Should be removed if upgrading to latest kernel/VirtualBox versions
helps.

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

- Bump VM memory to 512MB (otherwise MariaDB fails to start up).

MAR-151: Added support for Debian 10 Buster to database role:

- Updated role reference documentaiton.
- Updated role meta information.
- Updated tests.
- Fix test that produces different outputs during invocation on
Stretch/Buster.

MAR-148: Disable Ansible's default of enforcing group names without dashes through Molecule configuration files:

The following links might be a useful read:

- https://github.com/ansible/ansible/issues/56930 (issues relaetd to
lack of docs, but many complaints and explanations within)
- https://groups.google.com/forum/#!topic/ansible-devel/bjAcM9ferIw

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

- Updated Molecule test configuration to include Debian 9 Stretch.
- Updated documentation.

MAR-129: Updated database role for new Molecule and Ansible versions:

- Fixed linting issues.
- Moved test variables into group_vars.
- Renamed the server which effectively tests the optional
parameters (mainly the backup).
- Updated tests to use correct mechanism for obtaining the list of
hosts on top of which the tests should be run.

MAR-128: Upgraded tests for database 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).
- Updated backup_server and backup_client configuration in order to
make the backup testing more generic (and easier to update for
future).