Fixes #5223 - Fix migration of rudder-webapp to 2.11 on RPM

Fixes #5223 - Fix migration of rudder-webapp to 2.11 on RPM

cf http://www.rudder-project.org/redmine/issues/5223

The migration removed the file /opt/rudder/jetty7/contexts/rudder.xml since it was not owned by package.

I've built a package to try the patch and it worked
It is explained in the documentation (http://www.rpm.org/max-rpm-snapshot/s1-rpm-inside-files-list-directives.html) that we need to use a dummy file in the buildroot:
"The way to achieve this, is to use the %ghost directive. By adding this directive to the line containing a file, RPM will know about the ghosted file, but will not add it to the package. However it still needs to be in the buildroot."