distDocker:getImages is not useful

[cg2v]

distDocker's getImages returns the union of all the available images on all the backends, but distDocker doesn't assign vms to only hosts that have the desired image on them. Strictly speaking, getImages should return the intersection of the sets of available images on the backends, but in reality, I think maybe we should do some caching, or only check one backend per call to getImages.