Move image pull throttling logic to kubelet/images

This allows runtimes in different packages (dockertools, rkt, kuberuntime) to
share the same logic. Before this change, only dockertools support this
feature. Now all three packages support image pull throttling.

pkg/kubelet/kuberuntime/kuberuntime_image.go

@@ -28,7 +28,6 @@ import (
 
 // PullImage pulls an image from the network to local storage using the supplied
 // secrets if necessary.
-// TODO: pull image with qps and burst, ref https://github.com/kubernetes/kubernetes/blob/master/pkg/kubelet/dockertools/docker.go#L120
 func (m *kubeGenericRuntimeManager) PullImage(image kubecontainer.ImageSpec, pullSecrets []api.Secret) error {
 	img := image.Image
 	repoToPull, _, _, err := parsers.ParseImageName(img)