MAR-181: Drop support Debian 9 Stretch from mail_forwarder role:

- Switch to using IPs from VirtualBox default allowed host-only
network subnets.
- Use Debian Buster for helper machines.

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

- Updated role reference documentaiton.
- Updated role meta information.
- Updated tests.
- Set the smtpd_relay_restrictions configuration option for Postfix
SMTP server in mail_forwarder role (required for version found in
Debian 10 Buster).

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-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.

MAR-128: Upgraded tests for mail_forwarder 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.
- Fixed some linting issues.
- Rearranged IP addresses for test instances a bit.
- Expanded on available hosts for testing all OS/paramter
combinations.
- Switched to using a more resilient way to populate /etc/hosts during
prepartion phase.
- Expanded connectivity tests to include all the introduced hosts.