Update ML2 section of networking-guide
Update ML2 section of networking-guide

Change-Id: I98cb8e215dec13fe155940ebca4231db0407ea39

openstack/openstack-manuals,AlekhyaMallina-Vedams/openstack-manuals,AlekhyaMallina-Vedams/openstack-manuals,AlekhyaMallina-Vedams/openstack-manuals,openstack/openstack-manuals,openstack/openstack-manuals,openstack/openstack-manuals