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!
LOG.warning('Unable to update resource type "%s", id "%s"' % (resource_type, resource_id))
        return
    LOG.info('Updating resource %s of type %s' % (resource_id, resource_type))
    props = resource['Properties']
    if resource_type == 'Lambda::Function':
        client = aws_stack.connect_to_service('lambda')
        keys = ('FunctionName', 'Role', 'Handler', 'Description', 'Timeout', 'MemorySize', 'Environment', 'Runtime')
        update_props = dict([(k, props[k]) for k in keys if k in props])
        update_props = resolve_refs_recursively(stack_name, update_props, resources)
        if 'Code' in props:
            client.update_function_code(FunctionName=props['FunctionName'], **props['Code'])
        return client.update_function_configuration(**update_props)
    if resource_type == 'ApiGateway::Method':
        client = aws_stack.connect_to_service('apigateway')
        integration = props.get('Integration')
        
        kwargs = {
            'restApiId': props['RestApiId'],
            'resourceId': props['ResourceId'],
            'httpMethod': props['HttpMethod'],
            'requestParameters': props.get('RequestParameters')
        }
        if integration:
            kwargs['type'] = integration['Type']
            kwargs['integrationHttpMethod'] = integration.get('IntegrationHttpMethod')
            kwargs['uri'] = integration.get('Uri')
            return client.put_integration(**kwargs)
        kwargs['authorizationType'] = props.get('AuthorizationType')
        return client.put_method(**kwargs)
def fix_account_id_in_arns(params):