incremental image snapshot

Currently nova support create snapshot for instance. For each snapshot, nova
will take a full-copy of instance's image. The full-copy will waste a lot of
network bandwidth and storage. This BP propose to support incremental snapshot
of instance's image.

Nova will snapshot the instance incremental and upload the incremental image to glance,
glance will add parent_image_ref in metadata,
When booting instance with image, glance can check the image's property
'parent_image_ref', and then can find out all the parent image to combine them
as a whole image.

This idea is inspired by https://review.openstack.org/#/c/106306/2