mgr/cephadm: Add strings to assert statements

mgr/cephadm: Add strings to assert statements

This helps with: https://tracker.ceph.com/issues/48981

Looks like there is an assert somewhere:

```
Error EINVAL: Traceback (most recent call last):
  File "/usr/share/ceph/mgr/mgr_module.py", line 1269, in _handle_command
    return self.handle_command(inbuf, cmd)
   ...snip...
  File "/usr/share/ceph/mgr/orchestrator/module.py", line 550, in _list_services
    raise_if_exception(completion)
  File "/usr/share/ceph/mgr/orchestrator/_interface.py", line 653, in raise_if_exception
    raise e
AssertionError
```

Signed-off-by: Sebastian Wagner <sebastian.wagner@suse.com>