support create volume from instance snapshot image

If a instance boot from volume, snapshot the instance will create a image in glance and create a volume snapshot in cinder. The image size is 0, it just a link to the volume snapshot. When the user create volume from the snapshot image, in the current version, it will create a blank volume. so the user is troubled. This BP intends to improve the interface.
The scheme is as follows:
when the user create volume from snapshot image, the api create interface will parse out the snapshot image corresponds to the volume snapshot id. Then create a volume from the volume snapshot. So, the user can use this way to restore the instance's bootable volume.
I have Registered a bug https://bugs.launchpad.net/cinder/+bug/1560867. someone think should register a BP. so I register one that may be better for much disscusion.
the code review link is: https://review.openstack.org/#/c/314944/