Add a Domain to WorkflowExecution objects

This will be needed later for finding an activity type for instance.

moto/swf/models/__init__.py

@@ -148,7 +148,7 @@ class SWFBackend(BaseBackend):
         wf_type = domain.get_type("workflow", workflow_name, workflow_version)
         if wf_type.status == "DEPRECATED":
             raise SWFTypeDeprecatedFault(wf_type)
-        wfe = WorkflowExecution(wf_type, workflow_id,
+        wfe = WorkflowExecution(domain, wf_type, workflow_id,
                                 tag_list=tag_list, **kwargs)
         domain.add_workflow_execution(wfe)
         wfe.start()

moto/swf/models/workflow_execution.py

@@ -39,7 +39,8 @@ class WorkflowExecution(object):
         "CancelWorkflowExecution"
     ]
 
-    def __init__(self, workflow_type, workflow_id, **kwargs):
+    def __init__(self, domain, workflow_type, workflow_id, **kwargs):
+        self.domain = domain
         self.workflow_id = workflow_id
         self.run_id = uuid.uuid4().hex
         # WorkflowExecutionInfo