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': 'create_state_machine',
            'parameters': {
                'name': ['StateMachineName', PLACEHOLDER_RESOURCE_NAME],
                'definition': 'DefinitionString',
                'roleArn': lambda params, **kwargs: get_role_arn(params.get('RoleArn'), **kwargs)
            }
        }
    },
    'StepFunctions::Activity': {
        'create': {
            'function': 'create_activity',
            'parameters': {
                'name': ['Name', PLACEHOLDER_RESOURCE_NAME],
                'tags': 'Tags'
            }
        }
    },
    'SNS::Subscription': {
        'create': {
            'function': 'subscribe',
            'parameters': {
                'TopicArn': 'TopicArn',
                'Protocol': 'Protocol',
                'Endpoint': 'Endpoint'
            }
        }
    }
}
def convert_acl_cf_to_s3(acl):