Merge branch '2.1' into 2.2

# Conflicts:
#	gns3server/schemas/node.py

gns3server/controller/project.py

@@ -939,7 +939,12 @@ class Project:
         Start all nodes
         """
         pool = Pool(concurrency=3)
+        emit_warning = True
         for node in self.nodes.values():
+            if node.node_type == "traceng" and emit_warning:
+                self.controller.notification.emit("log.warning", {"message": "TraceNG nodes must be started one by one"})
+                emit_warning = False
+                continue
             pool.append(node.start)
         yield from pool.join()