Add SSH provider

[filoozom]

I know this is probably never going to get merged, but I still wanted to open a PR for reference, or in case someone is in need of something similar.

This is still a work in progress, and here is a non-exhaustive list of things I want to add:

• Add a check to make sure that the OS is Ubuntu (tested on 18.04 and 20.04)
• Copy the SSH key created by the CLI to the darknode user
• Maybe add password login, although this seems needlessly insecure

The current version of this draft seems to work well enough.