Automatic merge from submit-queue

ClientConfig should not default to http://localhost:8080

This changes clientcmd to skip the default cluster, but preserves the
behavior in kubectl. This prevents the possibility of an administrator
misconfiguration in kubelet or other server component from allowing a
third party who can bind to 8080 on that host from potentially
impersonating an API server and gaining root access.

@mikedanese @deads2k this removes the defaulting of http://localhost:8080 for server from everything except kubectl.



```release-note
Kubernetes server components using `kubeconfig` files no longer default to `http://localhost:8080`.  Administrators must specify a server value in their kubeconfig files.
```