Given a CF template that defines outputs as named exports, I expect the "describe-stack" endpoint to include the exported names. With localstack, that is currently not the case.

Example template:
```yaml
  Outputs:
    VPCId:
      Value: {Ref: MyVPC}
      Export:
        Name: My-VPCId

  Resources:
    MyVPC:
      Properties:
        CidrBlock: 10.0.0.0/16
      Type: AWS::EC2::VPC

```

AWS:
```
$ aws cloudformation describe-stacks
...
                {
                    "OutputKey": "VPCId",
                    "OutputValue": "vpc-0123456789",
                    "ExportName": "My-VPCId"
                },

```

localstack:
```
$ aws cloudformation --endpoint-url=http://localhost:4581/ describe-stacks
...
                {
                    "OutputKey": "VPCId",
                    "OutputValue": "vpc-0123456789"
                },
...
```
s3_client = aws_stack.connect_to_service('s3')
    buckets = s3_client.list_buckets()
    for bucket in buckets['Buckets']:
        if bucket['Name'] == name:
            return True
def queue_exists(name):
    sqs_client = aws_stack.connect_to_service('sqs')
    queues = sqs_client.list_queues()
    try:
        url = name if '://' in name else aws_stack.get_sqs_queue_url(name)
    except Exception:
        return False
    for queue_url in queues['QueueUrls']:
        if queue_url == url:
            return True
def topic_exists(name):