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
I feel that this link should not be created in the package, but rather in the postinst *if* the %{rudderdir}/jetty7/ directory exists. If it doesn't, this means the system jetty is installed, and the %{rudderdir}/jetty7/ hierarchy shouldn't exist.

If you agree, could you move it? (and rebase this PR too, it's no longer mergeable)