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!
'create': {
            'function': 'put_gateway_response',
            'parameters': {
                'restApiId': 'RestApiId',
                'responseType': 'ResponseType',
                'statusCode': 'StatusCode',
                'responseParameters': 'ResponseParameters',
                'responseTemplates': 'ResponseTemplates'
            }
        }
    },
    'Kinesis::Stream': {
        'create': {
            'function': 'create_stream',
            'parameters': {
                'StreamName': 'Name',
                'ShardCount': 'ShardCount'
            },
            'defaults': {
                'ShardCount': 1
            }
        },
        'delete': {
            'function': 'delete_stream',
            'parameters': {
                'StreamName': 'PhysicalResourceId'
            }
        }
    },
    'StepFunctions::StateMachine': {