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.