Return what is supported by a compute node

Ref https://github.com/GNS3/gns3-gui/issues/1448

gns3server/compute/builtin/__init__.py

@@ -34,3 +34,10 @@ class Builtin(BaseManager):
     def __init__(self):
 
         super().__init__()
+
+    @classmethod
+    def node_types(cls):
+        """
+        :returns: List of node type supported by this class and computer
+        """
+        return ['cloud', 'nat', 'ethernet_hub', 'ethernet_switch']