Validators should expose their id and severity

Validators should expose their id and severity (maybe from service properties, but haven't to be service properties at all) by their own:

[...]
public interface Validator <T> {

[...]

    String getId(); // validator.id

    int getSeverity(); // validator.severity

[...]
}

This addition allows less code in ValidationModelRetrieverImpl and makes ValidatorAndSeverity obsolete.