MAR-150: Use fixtures for X.509 artefacts in the mail_forwarder role:

- Removed the statically generated artefacts.
- Generate X.509 artefacts for tests using Gimmecert.
- Updated paths to point to generated artefacts.
- Introduced cleanup playbook for removing generated artefacts.

MAR-162: Make the smtp_relay_truststore parameter mandatory in mail_forwarder role:

- Dropped the defaults from mail_forwarder role.
- Updated group variables in role tests.
- Updated role reference documentation.
- Updated usage instructions to include the mandatory parameter.
- Deduplicated tests for the TLS files.

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

- Moved variables out of test playbook into group_vars.
- Fixed linting issues.
- Use top-level linting configuration file in tests.
- Updated how the hosts are referenced when executing the tests.