stubDomains should also support nameserver with port

[shashidharatd]

@bowei, Thanks for the feature to add nameserver to kubedns via stubDomains, its really useful.
I find that nameserver need to be pure ip and does not support appending with port.

it would be really useful to support nameserver appended with port, if somebody wants to deploy dns-server in kubernetes as a NodePort service and there could be possibilities where cloud-provider may not be supporting LoadBalancer type service.

We do have a use case in federation for federating clusters on non-cloud environments.

if agree, i can send a patch to support nameserver appended with port number

cc @kubernetes/sig-federation-misc