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.