datanode preflight dns check

[todvora]

During preflight, check that the provided datanode hostname is actually bound to an IP in the system. This is important because we need to be able to reach the datanode by using this hostname. Incorrectly configured DNS may break the certificate provisioning and communication later during the setup.

Fixes #20503

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Refactoring (non-breaking change)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have read the CONTRIBUTING document.
• I have added tests to cover my changes.

[moesterheld]

one minor change, otherwise looks good.
I am not too convinced that the check is super-meaningful since it might not be related to the dns used to resolve the name from Graylog or the other data nodes, but at least a simple user error can be caught.

[todvora]

@moesterheld You are right, it won't cover any network related problems, but it will catch a simple user configuration problem. Not overly helpful, but in the same time pretty simple implementation, so I hope we'll discover some problems even before they become big and hard to debug.