No support for `ceph config-key`

Sometimes you need to have prometheus scrape more targets.  For this you need to use e.g.

```plaintext
ceph config-key get mgr/cephadm/services/prometheus/prometheus.yml
```

```plaintext
ceph config-key set mgr/cephadm/services/prometheus/prometheus.yml -i prometheus.yml
```

There's currently no support for this here.