Can not delete the image has been launched instance when use rbd

[赵明俊]

Description
===========
When use rbd as nova and glance store backend, create an instance will directly clone image from glance， but the image did not do flatten， so When a image boot an instance, the image can not be deleted because it is in use.

There is a glance image property named 'img_linked_clone', used to control the association of image and instance, but it can not use in libvirt. if flatten the image after instance created, the association of image and instance will be broken, image and instance will be independent of each other.

Steps to reproduce
==================
A chronological list of steps which will bring off the
issue you noticed:
1.use rbd as the nova and glance backend.
2.upload a raw image.
3.use this image create an instance
4.try to delete the image

Expected result
===============
the image will be deleted.

Actual result
=============
the image can not be deleted

Environment
===========

TripleO（newton)
devstack(pike) with rbd

[OpenStack Infra (hudson-openstack)]

Fix proposed to branch: master
Review: https://review.openstack.org/473362

[OpenStack Infra (hudson-openstack)]

Change abandoned by Stephen Finucane (<email address hidden>) on branch: master
Review: https://review.openstack.org/473362
Reason: This has been here for some time with no updates. Abandoning to clean up the review queue