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"
                },
...
```
stack = get_stack_details(stack_name)
            self.assertEqual(stack['StackStatus'], 'CREATE_COMPLETE')
        retry(check_stack, retries=3, sleep=2)
        
        assert bucket_exists('cf-test-bucket-1')
        assert queue_exists('cf-test-queue-1')
        topic_arn = topic_exists('%s-test-topic-1-1' % stack_name)
        assert topic_arn
        assert stream_exists('cf-test-stream-1')
        resource = describe_stack_resource(stack_name, 'SQSQueueNoNameProperty')
        assert queue_exists(resource['PhysicalResourceId'])
        
        tags = s3.get_bucket_tagging(Bucket='cf-test-bucket-1')['TagSet']
        self.assertEqual(tags, [{'Key': 'foobar', 'Value': aws_stack.get_sqs_queue_url('cf-test-queue-1')}])
        tags = sns.list_tags_for_resource(ResourceArn=topic_arn)['Tags']
        self.assertEqual(tags, [
            {'Key': 'foo', 'Value': 'cf-test-bucket-1'},
            {'Key': 'bar', 'Value': aws_stack.s3_bucket_arn('cf-test-bucket-1')}
        ])
        
        subs = sns.list_subscriptions()['Subscriptions']
        subs = [s for s in subs if (':%s:cf-test-queue-1' % TEST_AWS_ACCOUNT_ID) in s['Endpoint']]
        self.assertEqual(len(subs), 1)