mgr/cephadm: init attrs created by settattr()

to address the test falure caused by mypy:
```
mypy run-test: commands[0] | mypy --config-file=../../mypy.ini ansible/module.py cephadm/module.py mgr_module.py mgr_util.py orchestrator.py orchestrator_cli/module.py rook/module.py
test_orchestrator/module.py
cephadm/module.py: note: In member "_check_for_strays" of class "CephadmOrchestrator":
cephadm/module.py:596: error: "CephadmOrchestrator" has no attribute "warn_on_stray_hosts"
cephadm/module.py:596: error: "CephadmOrchestrator" has no attribute "warn_on_stray_services"
cephadm/module.py:599: error: "CephadmOrchestrator" has no attribute "warn_on_stray_services"
Found 3 errors in 1 file (checked 8 source files)
```
see also python/mypy#5719

Signed-off-by: Kefu Chai <kchai@redhat.com>

src/pybind/mgr/cephadm/module.py

@@ -327,6 +327,15 @@ def __init__(self, *args, **kwargs):
         super(CephadmOrchestrator, self).__init__(*args, **kwargs)
         self._cluster_fsid = self.get('mon_map')['fsid']
 
+        # for mypy which does not run the code
+        self.ssh_config_file = None  # type: Optional[str]
+        self.inventory_cache_timeout = 0
+        self.service_cache_timeout = 0
+        self.mode = ''
+        self.container_image_base = ''
+        self.warn_on_stray_hosts = True
+        self.warn_on_stray_services = True
+
         # for serve()
         self.run = True
         self.event = Event()