fix authentication issue during setup

```
W0803 01:46:29.410530   53470 update_cluster.go:350] Exported kubeconfig with no user authentication; use --admin, --user or --auth-plugin flags with `kops export kubeconfig`
[…]
W0803 01:54:07.680800   53632 validate_cluster.go:184] (will retry): unexpected error during validation: error listing nodes: Unauthorized
```

Solution based on:
testground#67

Author: cskiraly

k8s/01_install_k8s.sh

@@ -53,7 +53,7 @@ fi
 
 kops create -f $CLUSTER_SPEC
 kops create secret --name $CLUSTER_NAME sshpublickey admin -i $PUBKEY
-kops update cluster $CLUSTER_NAME --yes
+kops update cluster $CLUSTER_NAME --yes --admin
 
 # Wait for worker nodes and master to be ready
 kops validate cluster --wait 20m