Adds support for L3 routing/NAT as a service plugin

- Adds L3 routing/NAT service plugin
- Removes L3 routing/NAT from ML2 plugin
- Moves "router:external" attribute to new extension "External-net"
- Introduces separate RPC topic for L3 callbacks from L3 agent

Implements: blueprint quantum-l3-routing-plugin

Change-Id: Id9af10c2910f9a1730b163203a68d101ffc3b282

neutron/plugins/plumgrid/plumgrid_plugin/plumgrid_plugin.py

@@ -27,6 +27,7 @@ from oslo.config import cfg
 from neutron.api.v2 import attributes
 from neutron.db import api as db
 from neutron.db import db_base_plugin_v2
+from neutron.db import external_net_db
 from neutron.db import l3_db
 from neutron.db import portbindings_db
 from neutron.extensions import portbindings
@@ -56,9 +57,10 @@ cfg.CONF.register_opts(director_server_opts, "PLUMgridDirector")
 
 class NeutronPluginPLUMgridV2(db_base_plugin_v2.NeutronDbPluginV2,
                               portbindings_db.PortBindingMixin,
+                              external_net_db.External_net_db_mixin,
                               l3_db.L3_NAT_db_mixin):
 
-    supported_extension_aliases = ["router", "binding"]
+    supported_extension_aliases = ["external-net", "router", "binding"]
 
     binding_view = "extension:port_binding:view"
     binding_set = "extension:port_binding:set"