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-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-158: Update default TLS ciphers configuration in the mail_server role:

- Updated the default value for parameter mail_server_tls_ciphers.
- Updated tests, making them explicitly test for enabled and disabled
ciphers.
- Refactored tests for TLS to use nmap ssl-enum-ciphers script for
listing available TLS versions and ciphers.
- Install nmap as part of preparation step.
- Updated role reference documentation.

MAR-162: Make the mail_ldap_tls_truststore, imap_tls_key, imap_tls_certificate, smtp_tls_key, and smtp_tls_certificate parameters mandatory:

- Dropped the defaults from mail_server role.
- Updated group variables in role tests.
- Changed the key/certificate file extensions to be more descriptive.
- Updated role reference documentation.
- Updated usage instructions to include the mandatory parameters.

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-152: Drop a couple of references to Debian 8 Jessie from multiple roles:

- Mark workaround as TODO entry (so maybe it can get fixed at some
point.
- Drop SSH known hosts entries in tests for backup_server role.
- Fix some erroneous comments in tests for mail_server
role (references to Debian 8 Jessie).
- Drop reference to Debian 8 Jessie from the preseed file template.

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.