Now there is no way to revert the pending-delete images. Once the
admin operators want to revert the delete action, Glance should
give them this kind of ability.
This patch will enhance the glance-scrubber tool to support restoring
the image from pending-delete to active.
Change-Id: I11fe58403e3af102b63d15b3cc702e567e526bad
blueprint: pending-delete-rollback