MAR-218: Fix/improve reliability of mail_server tests:

- The common role has rather conservative rate limit values that
easily get triggered when running connectivity tests in quick
succesions.

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-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.