MAR-28: Implemented additional tests for mail_server role:

- Deploy a number of tools on clients in order to test SMTP, IMAP, and Sieve
services.
- Added one more user to LDAP directory for testing group restrictions.
- Deploy CA certificate on all testing machines for TLS validation purposes.
- Use different custom-configured cipher for mail server ciphers.
- Fixed invalid postmaster address for parameters-optional host.
- Deploy configuration files for use with Imap-CLI on client test machines.
- Updated testing of SMTP server to include checks for users that do not belong
to mail group.
- Extended some SMTP-related tests to cover both test servers.
- Some small fixes in SMTP-related tests for expected output from commands.
- Implemented tests covering Dovecot (IMAP + Sieve) functionality.
- Implemented tests for running/enabled services.
- Implemented tests for ClamAV.
- Implemented tests for firewall and connectivity.
- Implemented tests for Postfix TLS configuration.
- TODO: Tests for Sieve TLS configuration have not been written yet due to
limitation of available tools.

MAR-28: Impelmented general and SMTP-related tests for mail_server:

- Increased amount of memory in test instances to 768MB.
- Fixed playbook for runs against client test instances.
- Fixed typos and invalid settings in test playbook.
- Switched to expanded syntax in tasks used for deploying TLS keys and
certificates in order to avoid mangling of TAB characters.
- Fixed missing permissions set-up for Postfix main configuration file.
- Fixed deployment of Postfix master.cf configuration file to take into account
configurable virtual mail user.
- Implemented tests covering SMTP functionality.

MAR-28: Implemented scaffolding for testing the mail_server role:

- Added Molecule configuration.
- Added test playbook.
- Restart Postfix for truststore changes.
- Added test data (private keys and certificates).
- Fixed small documentation inaccuracy.