Return Customer's Quota Usage through Admin API

As an admin user, I would like to know how much of a resource a customer has used, when that resource counts against a quota.

Customers can currently run "nova absolute-limits" to see what their quotas are and how much of each resource they have used (and, therefore, how much more they can use without having their quotas raised). Through admin API, we should be able to do "nova absolute-limits --tenant xxxxx" which should return the quota and the used limits for the given tenant.