Opencontrail network statistics driver

This patch introduces a network statistics driver
for Opencontrail. Only port statistics are currently
returned by the driver.

Implements: blueprint meter-from-opencontrail
Co-Authored-By: Edouard Thuleau <edouard.thuleau@cloudwatt.com>
Change-Id: Ic0afc478362fb4170903ee4e3723b82cd6c723fa
Merge "Opencontrail network statistics driver"