Support node label update

This PR is a basic support for node label update.  @brendanburns In case of label change, do we trigger a reschedule of current pods on updated node?  It's possible for the label change to break schedule predicate.  

@derekwaynecarr What is the status of node namespace.  Our api takes context obj, but I didn't see how it is used in k8s.  Are we going to only use default for node?   kubectl will check for namespace when updating, I'm not sure what namespace we'd use for node update.

PR #2412 validates that only labels are updated.