Send compute usage via in the compute.updated notification

Ref https://github.com/GNS3/gns3-gui/issues/1261

tests/handlers/api/controller/test_compute.py

@@ -127,8 +127,9 @@ def test_compute_list(http_controller, controller):
             'port': 84,
             'protocol': 'http',
             'user': 'julien',
-            'name': 'My super server'
-
+            'name': 'My super server',
+            'cpu_usage_percent': None,
+            'memory_usage_percent': None
         }
     ]