Allow delaying the actual deletion of an image

Allow delaying the actual deletion of an image.

If glance-api.conf has delayed_delete set to False or not set at all (it defaults to False) immediatly delete the image. If delayed_delete is set to True glance-api sets the status to pending_delete but does not call the backend delete. From the user's point of view it is actually deleted, and future calls will 404.

A new daemon glance-scrubber is introduced which looks for pending_delete images that are older than a configurable interval and will actually delete from the backend the images.