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.