New image status "deleting"

Glance image delete sets the status of image being deleted as deleted or pending_delete, it then proceeds to backend_store to
delete the image. This leads to inconsistency in image status if, any operation fails during execution. This fix introduces a new
status 'deleting'. For cases delayed_delete=False, first we set image status=deleting and then proceed for store delete. For cases where delayed_delete=True, image status is set to pending_delete as usual. In scrubber, when image is being deleted, set status=deleting. Post deleting in both cases, we set status=deleted and in case of any error status becomes error.