MAR-105: Added parameter for controlling firewall to mail_forwarder:

- Added new parameter smtp_from_relay_allowed.
- Updated role reference documentation.
- Added small note to usage instructions to mention the parameter's usability in
case of NAT'ed machines or laptops.
- Updated test playbook, adding another instance for testing the parameter, and
added tests that cover new parameter.
- Updated existing connectivity tests to be more specific and reliable.

MAR-27: Added initial scaffolding for testing mail_forwarder role:

- Fixed issues reported by Ansible linting check (some mode-related syntax and
one ignore.
- Added Molecule configuration for testing mandatory and optional
parameters. Covers both Debian Jessie and Debian Stretch.
- Added test playbook for setting-up the test instances. A helper relay mail
server.
- Updated both mail_server and mail_forwarder to fall-back to using
native (/etc/hosts) resolving if DNS fails. Solves issue with test environment
not having proper DNS set-up for all domains etc.
- Added a number of data/config files associated with tests.
- Added dummy test file.