Deprecate nova-network in favor of using Neutron

OpenStack Networking will reach full feature parity (and beyond) with nova-network in the Havana development cycle. To be able to officially deprecate nova-network and recommend OpenStack Networking, one of the last difficult problems to solve is how to migrate an existing deployment to OpenStack Networking. This will likely involve some code as well as some good documentation on how sample deployments can be migrated. This will be a join effort between the Nova and OpenStack Networking teams.