Add 'created_at' to execution() yaql function

Sometimes, an action needs to know the time passed since the whole
workflow started.

Change-Id: Id347e4cf2489f29dad7d16237d526660ee11665e

mistral/tests/unit/engine/test_yaql_functions.py

@@ -313,3 +313,8 @@ class YAQLFunctionsEngineTest(engine_test_base.EngineTestCase):
         )
 
         self.assertDictEqual({'param1': 'blablabla'}, execution['params'])
+
+        self.assertEqual(
+            wf_ex.created_at.isoformat(' '),
+            execution['created_at']
+        )