PICKED FROM TRUNK 21182

SCHEDULING-1438 Added the minimal probability for dynamic selection scripts after which they won't be executed on nodes.

git-svn-id: svn+ssh://scm.gforge.inria.fr/svn/proactive/scheduling/branches/Scheduling_3.1.x@21334 28e8926c-6b08-0410-baaa-805c5e19b8d6