Fixes #4989: Add rudder-plugin to the files provided with Rudder

Ticket: http://www.rudder-project.org/redmine/issues/4990

To be merged in 2.11
This all makes sense and looks good, except for one case: what if the <extraClasspath> tag is not already present in the file? In that case, your greps and seds will do nothing, and this will fail. I think you should check if it exists and add it if not.