Delete image when backup operation failed on snapshot step

when we snapshot an instance, we will use @delete_image_on_error to
delete any failed snapshot, however, the image will not be removed
by backup code flow, it will be an issue if too many backup failed
at last ,all useful image will be removed and we have only 'error'
image left in host

Change-Id: Ib527ade86aef52a8445b57fa03fed434578c4204
Closes-Bug: #1288574
Merge "Delete image when backup operation failed on snapshot step"