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.
@smarterclayton To clarify, you're proposing creating `github.com/GoogleCloudPlatform/kubernetes/pkg/kubelet/docker/`, correct?