cluster/kube-push.sh fails

https://github.com/kubernetes/kubernetes/blob/master/docs/devel/developer-guides/vagrant.md

I have a running cluster in vagrant, had to make some code changes and push back but `cluster/kube-push.sh` fails with:

```
$ ./cluster/kube-push.sh 
Updating cluster using provider: vagrant
./cluster/../cluster/../cluster/vagrant/util.sh: line 165: KUBE_BEARER_TOKEN: unbound variable
```
