Adds nova vif plugin for contrail/vrouter

OpenContrail System consists of two main components:
OpenContrail Controller and the OpenContrail vRouter.

- OpenContrail Controller is a logically centralized but physically distributed SDN controller
- OpenContrail vRouter is a forwarding plane (of a distributed router)
  that runs in the hypervisor of a virtualized server.
- This patch implements the nova vif driver for the vRouter.
- Corresponding neutron changes are at changeid
     I547426f1d4e0d1a305e179a785a5d517d058c381

Change-Id: Idef828f31b4d54f44f595c70aaea825cc2adf2cc
Implements: blueprint juniper-opencontrail-vrouter-nova-vif-driver
Closes-Bug: #1246837

Conflicts:

	nova/network/model.py
	nova/tests/virt/libvirt/test_libvirt_vif.py
	nova/virt/libvirt/vif.py