Hi, I'm trying to get LocalStack set up for integration testing and having a lot of trouble building CloudFormation templates that include nested stacks referenced via the `TemplateURL` property. Is this just not a thing that works in LocalStack? I've tried setting TemplateURL to a local file as well as to an object in an S3 bucket created with LocalStack.

For example if I call `create-stack` on a `template.yaml` containing the following:
```
Resources:
  SNSTopics:
    Type: AWS::CloudFormation::Stack
    Properties:
      TemplateURL: http://localhost:4572/artifacts/sns.yaml
```

...the stack builds but the referenced `sns.yaml` stack does not get built; whereas if I call `create-stack` directly on the referenced file it builds fine.

Thanks for any help/advice you can give!
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        subs = sns.list_subscriptions()['Subscriptions']
        subs = [s for s in subs if (':%s:cf-test-queue-1' % TEST_AWS_ACCOUNT_ID) in s['Endpoint']]
        self.assertEqual(len(subs), 1)
        self.assertIn(':%s:%s-test-topic-1-1' % (TEST_AWS_ACCOUNT_ID, stack_name), subs[0]['TopicArn'])
        
        test_api_name = 'test-api'
        api = [a for a in apigateway.get_rest_apis()['items'] if a['name'] == test_api_name][0]
        responses = apigateway.get_gateway_responses(restApiId=api['id'])['items']
        self.assertEqual(len(responses), 2)
        types = [r['responseType'] for r in responses]
        self.assertEqual(set(types), set(['UNAUTHORIZED', 'DEFAULT_5XX']))
        
        cf_client.delete_stack(StackName=stack_name)
        
        assert not bucket_exists('cf-test-bucket-1')
        assert not queue_exists('cf-test-queue-1')
        assert not topic_exists('%s-test-topic-1-1' % stack_name)
        retry(lambda: self.assertFalse(stream_exists('cf-test-stream-1')))
    def test_list_stack_events(self):
        cloudformation = aws_stack.connect_to_service('cloudformation')
        response = cloudformation.describe_stack_events()
        self.assertEqual(response['ResponseMetadata']['HTTPStatusCode'], 200)
    def test_validate_template(self):
        cloudformation = aws_stack.connect_to_service('cloudformation')
        template = template_deployer.template_to_json(load_file(TEST_TEMPLATE_1))
        response = cloudformation.validate_template(TemplateBody=template)
        self.assertEqual(response['ResponseMetadata']['HTTPStatusCode'], 200)
    def test_validate_invalid_json_template_should_fail(self):