Support for "usage" for "Cloud" nodes. Fixes https://github.com/GNS3/gns3-gui/issues/2887

Allow "usage" for all builtin nodes (not exposed in Ui).

gns3server/compute/dynamips/nodes/ethernet_hub.py

@@ -57,6 +57,7 @@ class EthernetHub(Bridge):
     def __json__(self):
 
         return {"name": self.name,
+                "usage": self.usage,
                 "node_id": self.id,
                 "project_id": self.project.id,
                 "ports_mapping": self._ports,