Opencontrail plug-in implementation for core resources

Adds Opencontrail plug-in implementation with unit tests
This patch has no dependency on any other blueprints
The link below describes how to install VIF driver for opencontrail
https://github.com/Juniper/contrail-controller/wiki/
OpenContrail-bring-up-and-provisioning

- The contrail_plugin_core.py is the main interface for neutron common
  infrastructure. It relays API requests to the opencontrail controller

DocImpact
Change-Id: I501bf669b2a999a171f9a3ee3e9893d4ead50e3b
Implements: blueprint juniper-plugin-with-extensions
Merge "Opencontrail plug-in implementation for core resources"