mgr/cephadm: Add example to run when debugging ssh failures

mgr/cephadm: Add example to run when debugging ssh failures

```
$ ceph orch host add foobar
Error ENOENT: Failed to connect to foobar (foobar).  Check that the host is reachable and accepts connections using the cephadm SSH key
you may want to run:
> ssh -F =(ceph cephadm get-ssh-config) -i =(ceph config-key get mgr/cephadm/ssh_identity_key) rook@foobar
$  ssh -F =(ceph cephadm get-ssh-config) -i =(ceph config-key get mgr/cephadm/ssh_identity_key) rook@foobar
ssh: Could not resolve hostname foobar: Temporary failure in name resolution
```

Signed-off-by: Sebastian Wagner <sebastian.wagner@suse.com>
(cherry picked from commit c840dd2204e9a419c28875ace18d33ad6f87500f)