Add a variable to configure the adress for remote debug in jetty start debug script

The current script start_xwiki_debug.sh provided in the jetty/hsqldb distribution of XWiki doesn't allow to configure easily the port for debugging operations.
Right now the port is hardcoded to use the standard 5005.

It's fine most of the time except when using two instances in debug at the same time for dev purposes: in that case the port must be changed in one of the instance to properly start XWiki.
We should provide a variable for it as we already do for JETTY_PORT and JETTY_STOP_PORT.