Add parent id to index image which has a backing file

The Glance is only store a image without backing file. The admin often build a simple image and the user setup some software in the instance. It is waste the user's time and also spend a lot of space. If the glance support the image with backing file. We can easily upload the copy-on-write image witch rely on a base image. It will allow the admin to make the image with different software in a operating system.