provide node path on configuration

| Q             | A
| ------------- | ---
| Doc fix?      | yes
| New docs? | No
| Applies to    | all
| Fixed tickets | 

If you no provide node path in assetic configuration, UglifyCssFilter throw a  `RuntimeException: Path to node executable could not be resolved.` 
The funny thing is that reST only complain about underlines that are too short.