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-163: Ensure host.run is not suspectible to shell injection:

- Use host.run's built-in capability for handling parameter escaping
and insertion.

MAR-152: Refactor tests for mail_forwader relay testing:

- Mark the helper machines in the Ansible inventory.
- Parametrise the tests to make adding future servers easier (e.g. do
not use specific hostnames in tests).
- Break-up relay test to be more specific (also to be able to
parametrise properly).

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.