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!
for resource_id, resource in iteritems(resources):
        if is_deployable_resource(resource):
            if not is_deployed(resource_id, all_resources, stack_name):
                return False
    return True
def resources_to_deploy_next(resources, stack_name):
    result = {}
    for resource_id, resource in iteritems(resources):
        if should_be_deployed(resource_id, resources, stack_name):
            result[resource_id] = resource
    return result
def get_resource_dependencies(resource_id, resource, resources):
    result = {}
    dumped = json.dumps(common.json_safe(resource))
    for other_id, other in iteritems(resources):
        if resource != other:
            
            search1 = '{"Ref": "%s"}' % other_id
            search2 = '{"Fn::GetAtt": ["%s", ' % other_id
            if search1 in dumped or search2 in dumped:
                result[other_id] = other
            if other_id in resource.get('DependsOn', []):
                result[other_id] = other
    return result