add an image_list tool

this is an interface from devstack to provide a more robust
listing of the possible image urls that we might ask for during
a devstack run. This allows tools like nodepool, or 3rd party CI
systems to precache all this content in advance.

Change-Id: I7474f553ecf28e51a2340bef5bcefa962dbadc24
Merge "add an image_list tool"