Return Customer's Quota Usage through Admin API

Modified the used limits API for Admin to retrieve
the used limits for a customer. Changes done to the
nova client to fetch limits for a given tenant.

This is related to changes made in nova,
https://review.openstack.org/#/c/27468/

Change-Id: Id53576eb35d6dab7cb655f8427091e95a6f75a6d
Implements: blueprint customer-quota-through-admin-api
Merge "Return Customer's Quota Usage through Admin API"