Don't use hardcoded flavors in tests

Use config values instead.

Change-Id: I5755ddc628e08a4336debef4022bebf1542b0200
diff --git a/functional/test_cancel_update.py b/functional/test_cancel_update.py
index f6ddc07..bfeeda6 100644
--- a/functional/test_cancel_update.py
+++ b/functional/test_cancel_update.py
@@ -53,7 +53,7 @@
 
         stack_identifier = self.stack_create(template=self.template,
                                              parameters=parameters)
-        parameters['InstanceType'] = 'm1.large'
+        parameters['InstanceType'] = self.conf.instance_type
         self.update_stack(stack_identifier, self.template,
                           parameters=parameters,
                           expected_status='UPDATE_IN_PROGRESS')