[Docs] Exception for firewalld config
[Docs] Exception for firewalld config

This patch refers deployers to more documentation and provides
recommendations for more specific firewall rules.

Implements: blueprint security-rhel7-stig
Change-Id: Ie6601b3038256e067e2307fd86ae9d6812c07783

major/openstack-ansible-security,major/openstack-ansible-security