Automatic merge from submit-queue (batch tested with PRs 44741, 44853, 44572, 44797, 44439)
add tests for managed hosts file content
**What this PR does / why we need it**: a tiny refactor to separate concerns host file content from the logic ensuring that the file exists, and also adds unit test.
This is to prepare extending the host file logic to include customized entries described in https://github.com/kubernetes/kubernetes/issues/43632. I'm wary of changing that logic in absence of any unit tests.
**Release note**:
```release-note
NONE
```
9dde8ec68c