PICKED FROM TRUNK 21302

(with conflict resolution)

SCHEDULING-1455 Added the possibility to specify particular users/groups for node source administration and utilization.

git-svn-id: svn+ssh://scm.gforge.inria.fr/svn/proactive/scheduling/branches/Scheduling_3.1.x@21338 28e8926c-6b08-0410-baaa-805c5e19b8d6