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!
'parentId': 'ParentId'
            }
        }
    },
    'ApiGateway::Method': {
        'create': {
            'function': 'put_method',
            'parameters': {
                'restApiId': 'RestApiId',
                'resourceId': 'ResourceId',
                'httpMethod': 'HttpMethod',
                'authorizationType': 'AuthorizationType',
                'requestParameters': 'RequestParameters'
            }
        }
    },
    'ApiGateway::Method::Integration': {
    },
    'ApiGateway::Deployment': {
        'create': {
            'function': 'create_deployment',
            'parameters': {
                'restApiId': 'RestApiId',
                'stageName': 'StageName',
                'stageDescription': 'StageDescription',
                'description': 'Description'
            }
        }
    },
    'ApiGateway::GatewayResponse': {