MAR-192: Added support for Debian 12 Bookworm to mail_server role:

- Allow use of older versions of TLS on Bookworm for testing of
related optional parameters.
- Install the IMAP CLI testing tool under its own virtual environment
in order to avoid warnings and errors when trying to install it
globally under Debian 12 Bookworm.
- Update the lists of expected TLS protocols and ciphers.

MAR-192: Update compatibility-related entries for Postfix main configuration file:

- Default value for append_dot_mydomain has been "no" for a while now,
no need to set it explicitly.
- Prefer the whitelist/blacklist instead of allowlist/denylist
keywords in logs.

MAR-196: Simplify allowed TLS protocol configuration for mail_server role:

- Accept minimum version allowed instead of arbitrary list.
- Fixes deprecation warnings in Dovecot logs (ssl_protocols ->
ssl_min_protocol transition).

MAR-189: Added support for Debian 11 Bullseye to mail_server role:

- Shorten the backup client username so it would be under 32
characters.
- Switch all helper VMs to using the Debian 11 Bullseye as well.
- Drop the architecture suffix from hostnames.
- Update the hostname for client VMs.
- Fix the incorrect format for the message ID used in various mail
delivery tests.

MAR-181: Drop support for Debian 9 Stretch from mail_server role:

- Switch to using IPs from VirtualBox default allowed host-only
network subnets.
- Use Debian Buster for helper machines.
- Drop Stretch-specific code and tests.

MAR-151: Use 2048-bit DH parameter for IMAP server under Debian 10 Buster:

- Deploy a statically-generated DH parameter.
- Set-up DH parameter configuration based on Debian version.
- Implemented test for newly-generated file.

MAR-151: Fix tests that fail due to differences between Debian Stretch and Debian Buster:

- Update the regex patterns used to locate deliveries via Dovecot.
- Enable verbose mode for gnutls-cli in one of the tests in order to
show the DH key size.
- Update the list of expected TLS ciphers for SMTP port 25 to account
for inclusion of additional ciphers in Debian Buster.
- Fix how the allowed relay IP is being fetched, because
host.ansible.get_variables method fails to resolve dynamic
variables.

MAR-163: Deduplicate test for certificate validity check configuration in mail_server role:

- Tests are already identical, they were probably kept separate by
mistake.

MAR-175: Mail server should be opportunistic in using TLS when delivering mail to remove servers:

- Previously the mail server would only deliver mails over plaintext.
- Deploy a simple SMTP server on both client1/client2
machines. Servers are set-up to require/refuse the STARTTLS over
SMTP.
- Added tests for checking if STARTTLS is used when available for mail
delivery.
- Fixed the wrong configurtion (making sure the TLS security level is
properly set for Postfix).

MAR-150: Use fixtures for X.509 artefacts in the mail_server 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.
- Increase allocated RAM for the mail server to avoid OOM and
swapping.

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-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.