Fix showing default help (from index.md) (#3205)

In case user executes:
- `tns help` - we should show the default index.html help with all commands
- `tns --help` - we should show the default command line help generated from index.md file with all commands

This has been broken during changing html-help-service to help-service.
Fix the default behavior and add tests for it.

Author: rosen-vladimirov