Async image bit deletion from specific store

The glance v2 API supports multiple locations per image which permits images to be backed on secondary storage locations. However from a deletion perspective, glance only supports deleting an image and ALL locations from an in-band perspective (single location deletion is an out-of-band operation). Moreover image deletion may take a considerable amount of time (depending on storage, image size, etc.) which is currently done in a synchronous fashion.

Ideally glance v2 would permit a user to initiate an async task to delete an image from a specific store backend location. Such support would allow operators to better and more effectively manage multiple image locations in-band.

This blueprint intends to outline an approach for providing a glance async task capable of deleting an image from a specific location. It seeks to address the high level use case:

"As an operator, I want delete image bits from specified locations in an effective manner."