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!
client = aws_stack.connect_to_service('cloudformation')
    try:
        result = client.describe_stack_resource(StackName=stack_name, LogicalResourceId=logical_resource_id)
        return result['StackResourceDetail']
    except Exception as e:
        LOG.warning('Unable to get details for resource "%s" in CloudFormation stack "%s": %s' %
                    (logical_resource_id, stack_name, e))
def retrieve_resource_details(resource_id, resource_status, resources, stack_name):
    resource = resources.get(resource_id)
    resource_id = resource_status.get('PhysicalResourceId') or resource_id
    if not resource:
        resource = {}
    resource_type = get_resource_type(resource)
    resource_props = resource.get('Properties')
    try:
        if resource_type == 'Lambda::Function':
            resource_id = resource_props['FunctionName'] if resource else resource_id
            return aws_stack.connect_to_service('lambda').get_function(FunctionName=resource_id)
        elif resource_type == 'Lambda::Version':
            name = resource_props.get('FunctionName')
            if not name:
                return None
            func_name = aws_stack.lambda_function_name(name)