Don't check availability of port -1 in launcher

The method org.nuxeo.launcher.config.ConfigurationGenerator.checkPortAvailable checks that a port is available during launch so we don't try to bind to a port that is already in use.
When using 0 or -1 (to disable some things), this check should be skipped.

See: http://answers.nuxeo.com/questions/5258/disabling-tomcat-shutdown-port