[Docs] Refer to other control for firewalld
[Docs] Refer to other control for firewalld

This patch adds docs that refer to another control for firewalld.

Implements: blueprint security-rhel7-stig
Change-Id: I57fbc64931ae4918a5db6750bd7e8ea1d3d4a01c

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