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

- Include an example of IPv6 address in documentation for the
smtp_allow_relay_from role parameter (has to have square brackets to
make it usable with Postfix configuratin).
- Expand the tests and test parameters to ensure IPv6 is properly
covered.
- Reorder the names in /etc/hosts slightly to ensure the ansible_fqdn
and inventory_hostname match-up. Code should be revisited to
probably drop the use of inventory_hostname from both roles and
tests, and instead rely on ansible_fqdn and ansible_hostname.
- Correct the Dovecot SSL DH parameter specification to use
ansible_fqdn for consistency purposes.

MAR-195: Reject mails with non-matching sender for logged-in users:

- Prevents logged-in user from setting "MAIL FROM" with e-mail address
that does not match either his own account or one of the aliases
that the user receives mails from.
- Reworked the submission port configuration to restrict only based on
sender-related restrictions, merging some of the newer changs from
the workaround.org instructions.

MAR-163: Refactor the connectivity test in mail_server role:

- Parametrise the test instead of looping within. Should help getting
cleaner error messages on what port has failed while also running
the test for every combination.
- Use safer way to pass arguments to host.run invocation.

MAR-163: Refactor the test_imap_authentication_requires_tls test:

- Parametrise the tests.
- Reduce code duplication.
- Make the host.run invocation slightly safer (currently it does not
matter, but it looks nicer this way).

MAR-148: Fixed tests for the mail_server role:

- Implement small workaround for sieve-connect invocations (due to
special treatment of 255 exit code by Testinfra itself).
- Fixed a couple of newline-related errors coming from command
outputs.

MAR-132: Added support for Debian 9 (Stretch) to mail_server role:

- Updated Molecule configuration to include set-up of additional
instances for testing.
- Updated configuration for test instances.
- Use separate clients in testing of Jessie/Stretch instances.
- Duplicate private keys/certificates for testing of mandatory
parameters on Debian 9.
- Refactored testing of mail deliveries (via swaks) to use
test-generated message ID - improves reliability and solves some
incompatibilities between swaks version in Jessie and Stretch.
- Updated tests for TLS testing to take into account newer OpenSSL
error/output messages. A bit of an ugly hack at the moment, but
beats duplicating tests for now.

MAR-129: Updated mail_server role for new Molecule and Ansible:

- Moved variables from test playbooks into group/host vars.
- Fixed linting errors.
- Reference custom top-level linting configuration file in tests.
- Fixed deprecation errors reported by Ansible.
- Updated how the hosts on which the tests should be run are
referenced.

MAR-128: Upgraded tests for mail_server 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.
- Renamed the hosts to make it easier to extend in future.
- Renamed some data files to ensure they correspond correctly to new
hostnames.
- Rewrote logic behind setting-up the /etc/hosts a bit, and expanded
the names to include the new hostnames.
- Updated a couple of tests to correspond to correct filenames (that
have been renamed).
- Updated a couple of tests to address the new server naming.
- Set explicitly the backup client username to avoid 32-charactr
limitation on Linux usernames.