This service plugin implements the L3 routing functionality (resources router
and floatingip) that in earlier releases before Havana was provided by core
plugins (openvswitch, linuxbridge, ... etc).

Core plugins can now choose not to implement L3 routing functionality and
instead delegate that to the L3 routing service plugin.

The required changes to a core plugin are in that case:
- Do not inherit 'l3_db.L3_NAT_db_mixin' (or its descendants like extraroute)
  anymore.
- Remove "router" from 'supported_extension_aliases'.
- Modify any 'self' references to members in L3_NAT_db_mixin to instead use
  'directory.get_plugin(constants.L3)'
  For example,
     self.prevent_l3_port_deletion(...)
  becomes something like
     plugin = directory.get_plugin(constants.L3)
     if plugin:
         plugin.prevent_l3_port_deletion(...)

If the core plugin has relied on the L3Agent the following must also be changed:
- Do not inherit 'l3_rpc_base.L3RpcCallbackMixin' in any '*RpcCallbacks' class.
- Do not be a consumer of the topics.L3PLUGIN topic for RPC.

To use the L3 routing service plugin, add
'neutron.services.l3_router.l3_router_plugin.L3RouterPlugin'
to 'service_plugins' in '/etc/neutron/neutron.conf'.
That is,
service_plugins = neutron.services.l3_router.l3_router_plugin.L3RouterPlugin
