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.


> On Sep 8, 2014, at 7:23 PM, Brian Waldon <notifications@github.com> wrote:
> 
> In pkg/kubelet/docker.go:
> 
> > @@ -57,14 +58,28 @@ type DockerPuller interface {
> >  
> >  // dockerPuller is the default implementation of DockerPuller.
> >  type dockerPuller struct {
> > -	client DockerInterface
> @smarterclayton To clarify, you're proposing creating github.com/GoogleCloudPlatform/kubernetes/pkg/kubelet/docker/, correct?
> 
Or similar name, it'll probably conflict with docker for the client so maybe dockertools or similar.
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