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.
New flag --container-runtime-endpoint (overrides --container-runtime)
is introduced to kubelet which identifies the unix socket file of
the remote runtime service. And new flag --image-service-endpoint is
introduced to kubelet which identifies the unix socket file of the
image service.