Add documentation to the firewall properties

miss documentation for the firewall properties,add it.

ref:
  http://docs.openstack.org/api/openstack-network/2.0/content/fwaas_ext_ops_firewall.html

Change-Id: I7bca7d29b05133d11ed53dede0caa561e792fa09
Merge "Add documentation to the firewall properties"