Fixes rbd _delete_image snapshot with missing image

Also ignore if _delete_image returns NotFound when cleaning up
following failed attempt to create a new image with add().

Also added unit tests.

Change-Id: Id66866b4260385a6324cc277c5ac665f81493c89
Fixes: bug 1233097
Merge "Fixes rbd _delete_image snapshot with missing image" into milestone-proposed