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.