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!
import os
import six
import logging
import traceback
from localstack import config
from localstack.services import install
from localstack.utils.aws import aws_stack
from localstack.constants import DEFAULT_PORT_ELASTICSEARCH_BACKEND, LOCALSTACK_ROOT_FOLDER
from localstack.utils.common import is_root, mkdir, chmod_r, rm_rf
from localstack.services.infra import get_service_protocol, start_proxy_for_service, do_run
from localstack.services.install import ROOT_PATH
LOG = logging.getLogger(__name__)
STATE = {}
def delete_all_elasticsearch_data():
    """ This function drops ALL data in the local Elasticsearch data folder. Use with caution! """
    data_dir = os.path.join(LOCALSTACK_ROOT_FOLDER, 'infra', 'elasticsearch', 'data', 'elasticsearch', 'nodes')
    rm_rf(data_dir)
def stop_elasticsearch():
    thread = STATE.get('_thread_')
    if not thread:
        return
    LOG.info('Terminating Elasticsearch instance, as all clusters have been removed')
    thread.stop()
def start_elasticsearch(port=None, delete_data=True, asynchronous=False, update_listener=None):