A volume-backed instance will not have the instance.image_ref
attribute set, to indicate to the API user that it is a volume-backed
instance.
Commit 984dd8ad6a missed this subtle
difference with how instance.image_ref is used, which means a rebuild
of any volume-backed instance will now run through the scheduler, even
if the image_href passed to rebuild is the same image ID as for the
root volume.
This fixes that case in rebuild by getting the image metadata off the
root volume for a volume-backed instance and compares that to the
image_href passed to rebuild.
Change-Id: I48cda813b9effa37f6c3e0cd2e8a22bb78c79d72
Closes-Bug: #1732947
54407afef3