At the moment, the module only allows to define the field prod_environment by
defining a `[delivery_carrier]` with `prod_environment` key as follows:

Restrict usage of prod environment:

  ```
  [delivery_carrier]
  prod_environment=False
  ```

Force usage of prod environment:

  ```
  [delivery_carrier]
  prod_environment=False
  ```

If the key is not set, the user will still be able to switch the value.