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 json
import unittest
from localstack.constants import TEST_AWS_ACCOUNT_ID
from localstack.utils.aws import aws_stack
from localstack.utils.common import load_file, retry, short_uid, to_str
from localstack.utils.cloudformation import template_deployer
from botocore.exceptions import ClientError
from botocore.parsers import ResponseParserError
THIS_FOLDER = os.path.dirname(os.path.realpath(__file__))
TEST_TEMPLATE_1 = os.path.join(THIS_FOLDER, 'templates', 'template1.yaml')
TEST_TEMPLATE_2 = os.path.join(THIS_FOLDER, 'templates', 'template2.yaml')
TEST_TEMPLATE_3 = """
AWSTemplateFormatVersion: 2010-09-09
Resources:
  S3Setup:
    Type: AWS::S3::Bucket
    Properties:
      BucketName: test-%s
""" % short_uid()
TEST_TEMPLATE_4 = """
AWSTemplateFormatVersion: 2010-09-09
Transform: AWS::Serverless-2016-10-31
Parameters:
  LambdaRuntime:
    Type: String