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-151: Added support for Debian 10 Buster to preseed role:

- Updated role reference documentaiton.
- Updated role meta information.
- Updated tests.
- Parametrise distribution release in one of the tests to cover both
servers with custom overrides being tested.

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-129: Updated preseed role for Ansible 2.5.x and new Molecule:

- Include top-level linting configuration in test configuration.
- Fixed linting errors.
- Moved test playbook variables into group_vars.
- Updated how hosts are referenced in tests.

MAR-128: Upgraded tests for preseed 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 hostname instead of hard-coding it in a
couple of tests.
- Introduced additional hosts for testing on different Debian
versions.