Fixes #4962: Enable Rudder integration with an eventual system jetty ins...

...tallation

Ticket: http://www.rudder-project.org/redmine/issues/4962

To be merged in 2.11
Shouldn't we move this directory to /opt/rudder/share/rudder-plugins too?

By "move" I mean "now create this directory, but still support both paths in all scripts to facilitate migration"