MAR-113: Added option for specifying relay port to mail_forwarder:

- Introduced new option "smtp_relay_host_port".
- Updated the test playbook and tests to make sure new functionality works as
expected.
- Update role reference documentation.
- Updated usage instructions.

MAR-27: Update mail_forwarder role/tests:

- Install swaks on mail-server instance for testing SMTP.
- Install procmail via mail_forwarder role (needed to be explicit for Debian
Stretch).
- Introduce small sleep when mails are sent to localhost for delivery to remote
hosts before checking the logs in order to allow Postfix to process the
queue.

MAR-27: Implemented tests for mail_forwarder role:

- Install hping3 (for testing connectivity) on mail-server test machine.
- Changed syntax used for deploying the SMTP relay truststore in order to ensure
that tabs are preserved.
- Implemented tests for the mail_fotwarder role.
- Updated tests for mail_server role to check for new setting
added (smtp_host_lookup).