Add support for the multiprovider API to ML2

This implements support for creating provider networks with
multiple different segments with the ML2 plugin.

Implements: blueprint ml2-multi-segment-api

Change-Id: Id6fc22841fddabfbe685de9605d4a4682ee1102d
ML2 Mechanism Driver for Cisco Nexus

Port of the quantum/plugin/cisco/nexus plugin to run under the Modular
Layer 2 (ML2) infrastructure as defined in
https://blueprints.launchpad.net/quantum/+spec/ml2-mechanism-drivers

Implements blueprint ml2-md-cisco-nexus

Change-Id: Ifdd03bec554a08266de859387f1901858a3be4a1
Merge "Add support for the multiprovider API to ML2"
Merge "ML2 Mechanism Driver for Cisco Nexus"