Don't load nested stack to get TemplateResource template

In the worst-case scenario if we can't find the template for an existing
TemplateResource, we fetch it from the extant nested stack. Rather than do
this by loading the nested stack in-memory, fetch the template via RPC.

Change-Id: Ide96ef861ef4cbb6afd9ab09e7b51b69e03e5520
Partial-Bug: #1731349

heat/engine/resources/template_resource.py

@@ -205,8 +205,11 @@ class TemplateResource(stack_resource.StackResource):
                 reported_excp = err
 
         if t_data is None:
-            if self.resource_id is not None:
-                t_data = jsonutils.dumps(self.nested().t.t)
+            nested_identifier = self.nested_identifier()
+            if nested_identifier is not None:
+                nested_t = self.rpc_client().get_template(self.context,
+                                                          nested_identifier)
+                t_data = jsonutils.dumps(nested_t)
 
         if t_data is not None:
             if t_data != stored_t_data:

heat/tests/test_provider_template.py

@@ -952,13 +952,12 @@ class TemplateDataTest(common.HeatTestCase):
 
     def test_template_data_in_create_without_template_file(self):
         self.res.action = self.res.CREATE
-        self.res.nested = mock.MagicMock()
-        self.res.resource_id = 'dummy_id'
+        self.res.resource_id = None
         self.res.get_template_file = mock.Mock(
             side_effect=exception.NotFound(
                 msg_fmt='Could not fetch remote template '
                         '"test_resource.template": file not found'))
-        self.assertEqual('{}', self.res.template_data())
+        self.assertRaises(exception.NotFound, self.res.template_data)
 
 
 class TemplateResourceCrudTest(common.HeatTestCase):