Supporting multiple SSH keys for a user

[slifty]

It's generally best practice to NOT have the same SSH key on multiple devices / generate one key per device, this means if we have developers with multiple machines they ought to have more than one SSH key associated with their user.

I imagine we can accomplish this by just adding more keys to the relevant /ssh/{user} file

We may want to document ssh expectations this in a /ssh/README.md or a /README.md