Arista ML2 Mechanism driver

This patch implements Arista's modular L2 mechanism driver to
automate the management of virtual networks along with physical networks
using Arista hardware devices (Spine and Leaf switches)

This driver uses ML2 Mechanism Driver-API to interface with Neutron ML2 Plugin.

implements: blueprint arista-ml2-mechanism-driver

Change-Id: I1c4ca36cf3d7af013b7b3353e7b7d89fe39f91bf