neutron api extension for service chaining

Currently Neutron does not support service chaining. To support service chaining, Service VMs must be attached at points in the
network and then traffic must be steered between these attachment points.

All Neutron network services and VMs are connected to a Neutron network via Neutron ports. This makes it possible to create a traffic steering model for service chaining that uses only Neutron ports. This traffic steering model has no notion of the actual services attached to these Neutron ports.

The network service function is instantiated and configured, and then it is connected to the network through Neutron ports. Once the service function is attached to Neutron ports, the ports may be included in a "port chain" to allow the service function to provide treatment to the user's traffic.