Update Configuration indicating that ws_coliposte_letter_service is required

Set ws_coliposte_letter_service node as required 

```
[Symfony\Component\Debug\Exception\ContextErrorException]                                                                                        
  Notice: Undefined index: ws_coliposte_letter_service in /var/www/gibertjoseph/logistic_outsourcing/www-logistic/src/vendor/lexik/colissimo-bundle/Lexik/Bundle/Coliss  
  imoBundle/DependencyInjection/LexikColissimoExtension.php line 28
```