Add a new line in nuxeo.conf to configure Jodconverter watch timeout

To help people configuring their preview, we should expose the system property directly in nuxeo.conf with a new line like

# Configure JODconverter watch timeout if you encounter difficulties with OpenOffice service startup
#JAVA_OPTS=$JAVA_OPTS -Djod.startup.watch.timeout=5

This will be helpful, like the line for people who wants to activate debugging or JMX access