"Foo" should not be an option to the "proxy_mode" variable.
Now the "proxy_mode" type is "choice", that means that there is a list of available values that can be selected.
We say that the "proxy_mode" variable is constrained (by choices): this variable accept, in fact, only a list of choices.
Now we can define the default value of this variable.
If the user doesn't touch this variable's value, the value is "No proxy"
As user intervention is no more required, so the variable change it's default mode too (from "basic" to "standard").
We can add a description to this first variable.
This information is useful for:
- documentation
- error message
- help user to known which value to set to this variable (for example with [the user data plugin Questionary](https://forge.cloud.silique.fr/gnunux/rougail-user-data-questionary)).
