[RHEL/7] Add initial firewalld XCCDF section

Initial firewalld dedicated XCCDF section based on information from:
[1] https://access.redhat.com/documentation/en-US/Red_Hat_Enterprise_Linux/7/html/Security_Guide/sec-Using_Firewalls.html
and
[2] https://fedoraproject.org/wiki/FirewallD

Please review.

Thank you.
Jan
Maybe say something like "For use on external networks with masquerading enabled such as on routers. Computers on the network are not trusted and could harm other computers."