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 re
import os
import json
import yaml
import base64
import logging
import traceback
import moto.cloudformation.utils
from six import iteritems
from six import string_types
from localstack.utils import common
from localstack.utils.aws import aws_stack
from localstack.utils.testutil import create_zip_file
from localstack.services.awslambda.lambda_api import get_handler_file_from_name
ACTION_CREATE = 'create'
ACTION_DELETE = 'delete'
PLACEHOLDER_RESOURCE_NAME = '__resource_name__'
LOG = logging.getLogger(__name__)
UPDATEABLE_RESOURCES = ['Lambda::Function', 'ApiGateway::Method']
NoDatesSafeLoader = yaml.SafeLoader
NoDatesSafeLoader.yaml_implicit_resolvers = {
    k: [r for r in v if r[0] != 'tag:yaml.org,2002:timestamp'] for
    k, v in NoDatesSafeLoader.yaml_implicit_resolvers.items()
}
def str_or_none(o):