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: 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).