Glance Quota Enhancements

At present there seem to be no way the operator can track information about the number of instance snapshot created. This is useful is the operator or the cloud provider wants to limit the amount of instance snapshot he wants to limit a particular user or tenant to. In Juno an option user_storage_quota was added which checks for available storage capacity (for non-zero values). In Icehouse it used to only support 0 (unlimited). We need something similar for checking the number of instance snapshots quota. Checking instance snapshot quota should be a preliminary step even before checking for available space. And if any of them fail the instance snapshot creation should bail out rather than proceeding ahead with uploading image to glance.

Also there are no CLI options to expose glance quota details, which IMHO makes it harder for end user to figure out what the quota options are and how it is being utilized.