Return node unhealthy if http request fails

If health check cannot talk to kubelet (http connection fail, instead of 500 code), it will return a connection fail instead of Unhealth status. Part of #2192

@lavalamp 
I agree that ErrNotHealthy is not appropriate for all cases; however, due to our current minion creation flow, we must return successful for unhealthy minion.  

I'd say instead of modifying health registry, I can remove the health check on CreateMinion:
https://github.com/GoogleCloudPlatform/kubernetes/blob/master/pkg/registry/minion/rest.go#L65