Document Neutron Firewall as a Service

This adds documentation for the Firewall as a Service (FWaaS)
feature being introduced in the Neutron Havana release.

Change-Id: I6cbd62ea295b21e28fa0b95c82d0ef1b46f47c93
Implements: blueprint neutron-fwaas-deployment
Merge "Document Neutron Firewall as a Service"