Make server.log logging level configurable via system property

Add <param name="Threshold" value="${jboss.server.log.threshold}"/> to the FILE appender config in jboss-log4j.xml.

Log4j performs a system property substitution (see org.apache.log4j.xml.DomConfigurator.subst()). It doesn't seem to support our default value syntax, e.g.

, but starting the AS w/o setting the system property results in DEBUG logging.

If we want a default other than DEBUG (or don't like counting on log4j to use DEBUG if the property isn't set), we can check the system property in Log4jService and set it if not set.