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 "ML2 Mechanism Driver for Cisco Nexus"