Add test on arbitrary output dict in action

Change-Id: I3cf0453638014fc53bd3005e434c6160f75d0cb0

mistral/tests/resources/data_flow/two_subsequent_tasks.yaml

@@ -21,7 +21,8 @@ Namespaces:
         parameters:
           - full_name
         output:
-          greeting: $
+          greeting:
+            greet_message: $
 
 
 Workflow:

mistral/tests/unit/engine/test_data_flow.py

@@ -304,10 +304,14 @@ class DataFlowTest(base.EngineTestCase):
                              build_greeting_task['parameters'])
         self.assertDictEqual(
             {
-                'greet_msg': 'Hello, John Doe!',
+                'greet_msg': {
+                    'greet_message': 'Hello, John Doe!'
+                },
                 'task': {
                     'build_greeting': {
-                        'greeting': 'Hello, John Doe!',
+                        'greeting': {
+                            'greet_message': 'Hello, John Doe!'
+                        },
                     }
                 }
             },