libvirt: Nova vif driver plugin for opencontrail

This blueprint is to add plugin for OpenContrail in existing Nova VIF driver to support, OpenContrail based network virtualization for Kilo release of Openstack.

The OpenContrail APIs will cover following:
* Create Interface
* Delete Interface
* Get Interface Config

Problem description
===================

OpenContrail is open source network virtualization solution. It uses standards
based BGP L3VPN closed user groups to implement virtual networks.
The link http://OpenContrail.org/OpenContrail-architecture-documentation/
explains the architecture of OpenContrail plugin

OpenContrail is loading its VIF driver via openstack-config command using option libvirt_vif_driver.
In Kilo this option is no longer supported and same needs to be implemented under Nova VIF driver.

Proposed change
===============

Add OpenContrail APIs to handle the Creation/Deletion/Get of interfaces in Nova VIF driver.
There are no changes to the Nova common code.

Implementation
==============

Vif Model - VIF_TYPE_VROUTER

Following APIs will be implemented:

def get_config_vrouter(self, instance, vif, image_meta, inst_type, virt_type)
def plug_vrouter(self, instance, vif)
def unplug_vrouter(self, instance, vif)

References
==========
http://www.OpenContrail.org
https://github.com/Juniper/contrail-controller