CLI: SSH Support about unit

[avahahn]

So, the general solution to this in my opinion is to open an SSH tunnel on demand for the user and then interact with the control API over it (or perhaps via SSHFS in the case of a Unix socket). At the moment the user can already configure either of those externally, but it would be a value add that Unitctl does this for them.

That said, I have qualms:

• do we want to be responsible for remote administration of a users server?
• do we really want to touch the user's ssh config?
• is the engineering time worth the effort for now? I am not aware of any other similar solution that does this (except maybe Teleport by Gravitational)

The reason why we are tracking this is to make sure we are at feature parity with unitc so that we can standardize on unitctl instead.

@lcrilly Im interested if you feel like this is a priority function or if you feel like we could replace unitc with this work still pending.