Fixes #4933: Display rudder server role in node details

The only real modification are in NodeInfo.scala, LDAPEntityMapper.scala (add an understand roles) and DisplayNode.scala (display them, trying to be somehow consistent with the time before roles). 

Other modification are cleaning around isPolicyServer not used (so that we don't have even more inconsistencies than between isPolicyServer and roles) and cleaning of dead code that would have had to be modified if not removed.