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!
if isinstance(o, dict):
            for k, v in dict(o).items():
                if v is None:
                    o.pop(k)
        return o
    result = common.recurse_object(params, remove_nones)
    return result
def deploy_resource(resource_id, resources, stack_name):
    return execute_resource_action(resource_id, resources, stack_name, ACTION_CREATE)
def delete_resource(resource_id, resources, stack_name):
    return execute_resource_action(resource_id, resources, stack_name, ACTION_DELETE)
def execute_resource_action(resource_id, resources, stack_name, action_name):
    resource = resources[resource_id]
    resource_type = get_resource_type(resource)
    func_details = RESOURCE_TO_FUNCTION.get(resource_type)
    if not func_details or action_name not in func_details:
        LOG.warning('Action "%s" for resource type %s not yet implemented' % (action_name, resource_type))
        return
    LOG.debug('Running action "%s" for resource type "%s" id "%s"' % (action_name, resource_type, resource_id))
    func_details = func_details[action_name]
    func_details = func_details if isinstance(func_details, list) else [func_details]
    results = []
    for func in func_details:
        if callable(func['function']):
            result = func['function'](resource_id, resources, resource_type, func, stack_name)
            results.append(result)
            continue
        client = get_client(resource, func)
        if client: