Cache image in cinder backend

When creating a bootable Cinder volume, we copy an image to a Cinder volume which comes from a Cinder backend. Next time we create a bootable Cinder volume with the same image, it needs to copy the image to the Cinder backend again. This BP is to implement a cache in Cinder backend so that when the first time customers create a bootable volume with an image, it creates a cached image copy in the Cinder backend. Then later when customers create bootable volumes with the same image, Cinder no need to copy an image from the image store.