mgr/cephadm: init attrs created by settattr()

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 https://github.com/python/mypy/issues/5719

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