Fix `minikube addons open heapster`
Fix `minikube addons open heapster`

In the default installation I get the same error:
```
$ minikube addons open heapster
💣  This addon does not have an endpoint defined for the 'addons open' command.
You can add one by annotating a service with the label kubernetes.io/minikube-addons-endpoint:heapster
```
This PR is simply implementing the suggested fix by adding the aforementioned label to the heapster service.
warmchang/minikube,dalehamel/minikube,kubernetes/minikube,kubernetes/minikube,warmchang/minikube,warmchang/minikube,dlorenc/minikube,warmchang/minikube,kubernetes/minikube,dlorenc/minikube,warmchang/minikube,dalehamel/minikube,dlorenc/minikube,kubernetes/minikube,kubernetes/minikube,dalehamel/minikube,warmchang/minikube,dlorenc/minikube,dlorenc/minikube,dlorenc/minikube,kubernetes/minikube,kubernetes/minikube,dlorenc/minikube,dlorenc/minikube,warmchang/minikube