mgr/cephadm: warn when draining host explicitly listed in placement

mgr/cephadm: warn when draining host explicitly listed in placement

In the case you apply a spec like

```
service_type: node-exporter
placement:
  hosts:
  - host3
```

and then you run `ceph orch host drain host3`, cephadm will remove
the daemon from that host and the placement would now match nothing.

This is definitely an issue that should be able to be bypassed as
it generally isn't serious, but it would be good to let users
know they have the host listed explicitly in placements like this
when they want to drain it.

Fixes: https://tracker.ceph.com/issues/63220
Signed-off-by: Adam King <adking@redhat.com>