add support for slack nebula

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  1. it uses the openvpn tap 6 driver.
  2. it does not have a gui and it seems there will not be a supported gui

https://arstechnica.com/gadgets/2019/12/nebula-vpn-routes-between-hosts-privately-flexibly-and-efficiently/
https://arstechnica.com/gadgets/2019/12/how-to-set-up-your-own-nebula-mesh-vpn-step-by-step/