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
Shouln't we just provide both of these? Since I can easily imagine someone installing a rudder-jetty RHEL6 RPM on a Fedora machine or vice-versa... And since the "other" one doesn't exist on each OS, I see no harm.