mgr/cephadm: fix flake8 test failures

mgr/cephadm: fix flake8 test failures

after a recent update on flake8, some tests in cephadm started failing
```
flake8: commands[0]> flake8 --config=tox.ini alerts balancer cephadm cli_api crash devicehealth diskprediction_local hello iostat localpool nfs orchestrator prometheus rbd_support rgw selftest smb
cephadm/configchecks.py:94:6: E204 whitespace after decorator '@'
cephadm/module.py:3017:10: E204 whitespace after decorator '@'
cephadm/module.py:3344:144: E226 missing whitespace around arithmetic operator
cephadm/tests/test_cephadm.py:982:6: E204 whitespace after decorator '@'
3     E204 whitespace after decorator '@'
1     E226 missing whitespace around arithmetic operator
```

Signed-off-by: Nizamudeen A <nia@redhat.com>