MAR-233: Add IPv6 connectivity tests for the mail_forwarder role:

- Configure static IPv6 addresses on all test machines.
- Parametrise all connectivity tests to cover both IPv4 and IPv6.
- Be explicit on receiving servers for all swaks invocations that
target localhost.
- Make sure that the helper mail server has the necessary iptables
forwarding rules for IPv6 as well.

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-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 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.