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.
Log what, the opposite of "Pulling image %s without credentials"? What log message would you like to see? I'd prefer not to log the user/pass.