FIX : replace publicIP (decapricated) for clusterIP

It fix this bug :
https://stackoverflow.com/questions/34255601/kube-addons-service-failed-on-coreos-libvirt-installation

Some documentation that explain why clusterIP is better :
https://github.com/kubernetes/kubernetes/blob/master/docs/api.md

cluster/libvirt-coreos/skydns-svc.yaml

@@ -6,7 +6,7 @@ metadata:
   name: skydns
   namespace: kube-system
 spec:
-  portalIP: ${DNS_SERVER_IP}
+  clusterIP: ${DNS_SERVER_IP}
   ports:
   - port: 53
     protocol: UDP