add banner for initial datanode start, promoting preflight interface

[todvora]

/nocl

We should notify users during initial datanode startup that the installation is not complete and it's waiting either for full SSL configuration or remote configuration via the preflight interface.
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Screenshots (if appropriate):

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.