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!
'ACL': lambda params, **kwargs: convert_acl_cf_to_s3(params.get('AccessControl', 'PublicRead')),
                'CreateBucketConfiguration': lambda params, **kwargs: get_bucket_location_config()
            }
        },
        'delete': {
            'function': 'delete_bucket',
            'parameters': {
                'Bucket': 'PhysicalResourceId'
            }
        }
    },
    'SQS::Queue': {
        'create': {
            'function': 'create_queue',
            'parameters': {
                'QueueName': ['QueueName', PLACEHOLDER_RESOURCE_NAME],
                'Attributes': lambda params, **kwargs: select_attributes(params,
                    ['DelaySeconds', 'MaximumMessageSize', 'MessageRetentionPeriod',
                     'VisibilityTimeout', 'RedrivePolicy']
                ),
                'tags': 'Tags'
            }
        },
        'delete': {
            'function': 'delete_queue',
            'parameters': {
                'QueueUrl': 'PhysicalResourceId'
            }
        }
    },