Make easier to configure Jetty in the xjetty Debian package

Right now you have to either modify /lib/systemd/system/xwiki.service (which require a reload of systemd) or /usr/lib/xwiki-jetty/start_xwiki.sh (which can hadly be called a configuration file).

It would be much cleaner if /usr/lib/xwiki-jetty/start_xwiki.sh was loading a file with various environment variables (something like the /etc/defaults/tomcat9 file).