Make load-image check image checksum

This change causes load-image to skip uploading an image if the
local file checksum already matches the checksum of the existing
image in glance.  This makes it possible to use load-image to
quickly retrieve the id of an existing image in Glance.

Change-Id: If706bd595d87922418bdfd48fed49207d32b8de8
Merge "Make load-image check image checksum"