Add support for creating provider networks

Implements blueprint neutron-provider-networking

Change-Id: I4bad9483b92875ea5e143b0e11652e01faed35a4
squash

Change-Id: Iccd331c1be90d4fb18014c871441f25252e269e0
Create an IPv6 network that uses an external router for IPv6

Change-Id: Id1498d74db816b3edbb8376ca5acb7cc5792ea5c