Modular L2 GRE Network Support

The ml2 plugin currently supports the flat, local, and vlan network types via TypeDrivers, using the openvswitch, linuxbridge, and hyperv agents. For parity with the openvswitch plugin, it needs to also support the gre network type. This involves a GreTypeDriver that will manage allocation of tunnel_ids, along with implementation of the tunnel endpoint management RPCs used by the openvswitch agent for GRE networks.

This implementation should also take into consideration the possibility that future network types such as vxlan (see https://blueprints.launchpad.net/quantum/+spec/ovs-vxlan-lisp-tunnel, https://blueprints.launchpad.net/quantum/+spec/vxlan-linuxbridge, and https://blueprints.launchpad.net/quantum/+spec/openvswitch-kernel-vxlan) may also require tunnel endpoint management RPC support (when multicast is not being used). Whether tunnel endpoints are a global resource, or are specific to each network type needs to be determined.

It should also take the https://blueprints.launchpad.net/quantum/+spec/ovs-tunnel-partial-mesh and https://blueprints.launchpad.net/quantum/+spec/l2-population blueprints into consideration, since these are likely to extend or modify the tunnel endpoint management RPC APIs and semantics.