Quotas for snapshots

It would be very helpful to be able to limit the utilization of some basic snapshot-related resources, like
- the number of snapshots one has uploaded to the storage
- the amount of storage one uses to store the snapshots
- etc

Apparently implementation of such limits would require Glance
- to include some quota handling routines similar to those in Nova and Cinder
- to make difference between the application-domain type of the images being stored

Since there is already a blueprint which involves quite similar requirements and thus potentially would have the same implementation it seems reasonable to add it as a dependency.