cephadm: fix `alertmanager not implemented yet`

cephadm: fix `alertmanager not implemented yet`

```
Traceback (most recent call last):
  File "/home/user/src/ceph/src/pybind/mgr/cephadm/module.py", line 391, in do_work
    res = self._on_complete_(*args, **kwargs)
  File "/home/user/src/ceph/src/pybind/mgr/cephadm/module.py", line 455, in call_self
    return f(self, *inner_args)
  File "/home/user/src/ceph/src/pybind/mgr/cephadm/module.py", line 2576, in _create_alertmanager
    custom_config=self._generate_alertmanager_config)
  File "/home/user/src/ceph/src/pybind/mgr/cephadm/module.py", line 2051, in _create_daemon
    stdin=json.dumps(cephadm_config))
  File "/home/user/src/ceph/src/pybind/mgr/cephadm/module.py", line 1459, in _run_cephadm
    code, '\n'.join(err)))
RuntimeError: cephadm exited with an error code: 1, stderr:ERROR: alertmanager not implemented yet
```

Signed-off-by: Patrick Seidensal <pseidensal@suse.com>