This adds a /v2/info/usage API endpoint which exposes to the user
their current limits and usage.
The discovery API does not (appear to) have existing tests, so this
adds a module for that, although only usage tests are added currently.
Implements: blueprint quota-api
Change-Id: I50c98bac50f815bdb9baae024e77afd388f74554
f865b8cac7