support the new devstack image finder

if the new devstack image_list.sh tool is found, use that are our
definitive image collection API out of devstack. We fall back to
the old method if it's not found, or fails to execute cleanly.

Change-Id: I9c59ac505b0a1085add76bfc6cb1b14e5545fc28
Merge "support the new devstack image finder"