add-volume-re-image-api

Currently, users can create a volume with a specific image, but once this
volume is created, the image of this volume can't be changed anymore. If users
want to re-image a specific volume, they can only create a new volume with new
image, and then delete the old volume, which is not very convenient.

In addition, as mentioned in nova bp "volume-backend-server-rebuild" `[1]`_,
if users want to rebuild a volume-backed server, they can complete it by using
re-image api, this way there is no new volume with new volume id, we don't
have to worry about types, quota problem.

So, we need a API to re-image a volume more conveniently.

_`[1]` http://review.openstack.org/#/c/532407