Do not strip 'glance://' prefix from image hrefs

pxe_utils.get_deploy_kr_info and drivers.modules.pxe._get_image_info
functions strip 'glance://' prefix from image references while it's
not necessary as Glance service can download images by such refs.
Also some refactoring done as these references are not just uuids.

Partially implements: blueprint non-glance-image-refs

Change-Id: Ie0d87c96c80dab23ea81011413600d9dbbf50177
Merge "Do not strip 'glance://' prefix from image hrefs"