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!
cloudformation = aws_stack.connect_to_service('cloudformation')
        
        stack_name = 'stack-%s' % short_uid()
        cloudformation.create_stack(StackName=stack_name, TemplateBody=TEST_TEMPLATE_3)
        
        response = cloudformation.create_change_set(StackName=stack_name,
            TemplateBody=TEST_TEMPLATE_3, ChangeSetName='nochanges')
        self.assertIn(':%s:changeSet/nochanges/' % TEST_AWS_ACCOUNT_ID, response['Id'])
        self.assertIn(':%s:stack/' % TEST_AWS_ACCOUNT_ID, response['StackId'])
    def test_sam_template(self):
        cloudformation = aws_stack.connect_to_service('cloudformation')
        awslambda = aws_stack.connect_to_service('lambda')
        
        stack_name = 'stack-%s' % short_uid()
        func_name = 'test-%s' % short_uid()
        template = TEST_TEMPLATE_4 % func_name
        cloudformation.create_stack(StackName=stack_name, TemplateBody=template)
        
        result = awslambda.invoke(FunctionName=func_name)
        result = json.loads(to_str(result['Payload'].read()))
        self.assertEqual(result, {'hello': 'world'})