When the cluster is external we want to expose manager service port
along with the endpoints.
This allows us to connect but Rook will keep on using 9283 as a facing
port for Prometheus and more.
```
monitoring:
enabled: true
...
...
externalMgrPrometheusPort: 9283
```
Signed-off-by: Sébastien Han <seb@redhat.com>