Desktop Linux support

As we all use Macs, Desktop Linux was not a fully fledged citizen of nscale.

This PR addresses this by:

1. Fixing `nsd server start|stop` command.
2. Remove completely all `sudo` commands.
3. If a permission problem is detected, hint to a possible solution.

Do not merge yet, it's here only for review. Before doing that we need to fix docker-container to not use `sudo`.

Thanks to @Analect for all the support in understanding the problem, and thanks to @mmalecki for pointing me to this very simple solution. 

Initially reported in: https://github.com/nearform/nscale/issues/7