Add config function to canonicalize the image name passed in.
This is strictly for the benefit of the Emulab config where you can request an image name by either ID or path, e.g. "testbed/FOO" or "/proj/testbed/images/FOO.ndz". Previously, simultaneous requests under both names would start up two different servers because we didn't know they were the same image. Not really a problem for downloads, but really not a good thing for uploads!