Scalable Neutron OVS agent: 1-to-N mapping

In current neutron deployment and design, the Neutron OVS ML2 agent is designed to manage the local single OVS running on the same host as the Neutron OVS agent. In a large scale deployment(i.e. more than 5000 nodes), this requires deploying more than 5000 Neutron OVS agents which doesn't scale well due to the message bus limitations.

Based on https://blueprints.launchpad.net/neutron/+spec/remote-ovs-agent, it's possible to make a single Neutron OVS agent manage multiple remote OVSs.

This bp describe the way to change exsiting 1-to-1 mapping between Neutron OVS agent and OVS to 1-to-N mapping, to make it more scalable.