Get SSH configuration from the .ssh/config file

I propose a somewhat significant change w.r.t. SSH configuration of a computer. Currently, we are duplicating the configuration found in .ssh/config. This configuration is not exportable, so it makes very little sense to store it in AiiDA.

Provided that, I propose to almost entirely remove SSH configuration in AiiDA, but instead automatically read the configuration from .ssh/config. This can guarantee that if ssh computer works, verdi computer test will work too. Additionally, the SSH setup will become minimal.