NET-1962: add gateway subcommand.

[VishalDalwadi]

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Checklist before requesting a review

• My changes affect only 10 files or less.
• I have performed a self-review of my code and tested it.
• If it is a new feature, I have added thorough tests, my code is <= 1450 lines.
• If it is a bugfix, my code is <= 200 lines.
• My functions are <= 80 lines.
• I have had my code reviewed by a peer.
• My unit tests pass locally.
• Netmaker is awesome.