setup-script overwrites old setup

[frissdiegurke]

when I run ./nodebb setup I expect only the settings to be changed where I type in sth (since empty values are saved ones).

But: If I change one of the settings that don't get asked (bcrypt_rounds, upload_path, use_port, relative_path) they get set to default value instead of staying the same.

[julian]

@frissdiegurke Good point, that's never been an issue for us as we always use the defaults for those settings.

I'm re-designing the install step at the moment, so will take this into account.