Add a storage quota

This patch adds a storage quota that is applied against the sum total
of a users storage consumption against all configured storage systems.
A single quota is applied to all users via the configuration option
'total_storage_quota'.  Most of the patch is about enforcement so
when a separate service for quota management emerges in OpenStack the
per user value to enforce can be obtained from that service but the
enforcement code will remain the same.

blueprint glance-basic-quotas
docImpact
Change-Id: I251832f7372c70942be6f0c6aa12285145dd7c18
Merge "Add a storage quota"