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-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-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.