Authenticated docker pulls, pt. I

The simplest reasonable thing to do here is to read the `.dockercfg` files that are already available to the kubelets. This only happens on startup, so any changes the user makes  to `.dockercfg` will only be applied after a service restart.
I haven't reached out to anyone in docker proper yet. I'd like to get something that works for us then propose moving it either into go-dockerclient or further upstream.