Use IPv4-only WSGI start up as a fallback only when the IPv6/IPv4 one didn't work

cps.py

@@ -30,7 +30,7 @@ if __name__ == '__main__':
         else:
             web.app.logger.info('Falling back to Tornado')
             http_server = HTTPServer(WSGIContainer(web.app))
-            http_server.listen(web.ub.config.config_port, '0.0.0.0')
+            http_server.listen(web.ub.config.config_port)
             IOLoop.instance().start()
             IOLoop.instance().close(True)
 

cps/web.py

@@ -3046,5 +3046,8 @@ def upload():
 def start_gevent():
     from gevent.wsgi import WSGIServer
     global gevent_server
-    gevent_server = WSGIServer(('0.0.0.0', ub.config.config_port), app)
+    try:
+        gevent_server = WSGIServer(('', ub.config.config_port), app)
+    except socket.error:
+        gevent_server = WSGIServer(('0.0.0.0', ub.config.config_port), app)
     gevent_server.serve_forever()