Although `kubectl attach POD_NAME <tab>` completes container name,
kubectl attach needs `-c` option so the command causes error as:
```
$ kubectl attach nginx-7cdbd8cdc9-b5rhr nginx
error: the server doesn't have a resource type "nginx-7cdbd8cdc9-b5rhr"
```
This patch changes the completion to the same way as `kubectl exec`.
f7aca76bac