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-163: Deduplicate tests for the preseed directory in preseed role:

- Run a single test on all three servers.
- Explicitly specify the preseed directory path (instead of using
constant).

MAR-155: Make the ansible_key parameter in the preseed role mandatory:

- Updated the preseed role.
- Removed default value for the parameter.
- Updated tests.
- Updated role reference documentation.
- Updated release notes.

MAR-155: Make the preseed_directory parameter in the preseed role mandatory:

- Updated the preseed role.
- Removed default value for the parameter.
- Updated tests.
- Updated role reference documentation.
- Updated release notes.

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-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.