KubeDNS Addon should uses kubedns instead of etcd/kube2sky

The KubeDNS addon doesn't use etcd + kube2sky anymore but kubedns. We should update the docs accordingly.
- https://github.com/kubernetes/kubernetes/tree/master/cluster/addons/dns
- https://github.com/kubernetes/kubernetes/commit/4c1047d359fa1ee64aa30508b5be4407aeec19a3.
