[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, "Computers on the network are not trusted and could harm your computer." I would probably avoid the use of 'you'.