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"
                },
...
```
self.assertIn(':%s:%s-test-topic-1-1' % (TEST_AWS_ACCOUNT_ID, stack_name), subs[0]['TopicArn'])
        
        attrs = sns.get_subscription_attributes(SubscriptionArn=subs[0]['SubscriptionArn'])['Attributes']
        self.assertEqual(attrs, {'Endpoint': subs[0]['Endpoint'], 'Protocol': 'sqs',
            'SubscriptionArn': subs[0]['SubscriptionArn'], 'TopicArn': subs[0]['TopicArn'],
            'FilterPolicy': json.dumps({'eventType': ['created']})})
        
        test_api_name = 'test-api'
        api = [a for a in apigateway.get_rest_apis()['items'] if a['name'] == test_api_name][0]
        responses = apigateway.get_gateway_responses(restApiId=api['id'])['items']
        self.assertEqual(len(responses), 2)
        types = [r['responseType'] for r in responses]
        self.assertEqual(set(types), set(['UNAUTHORIZED', 'DEFAULT_5XX']))
        
        cf_client.delete_stack(StackName=stack_name)
        
        assert not bucket_exists('cf-test-bucket-1')
        assert not queue_exists('cf-test-queue-1')
        assert not topic_exists('%s-test-topic-1-1' % stack_name)
        retry(lambda: self.assertFalse(stream_exists('cf-test-stream-1')))
    def test_list_stack_events(self):
        cloudformation = aws_stack.connect_to_service('cloudformation')
        response = cloudformation.describe_stack_events()
        self.assertEqual(response['ResponseMetadata']['HTTPStatusCode'], 200)
    def test_validate_template(self):
        cloudformation = aws_stack.connect_to_service('cloudformation')
        template = template_deployer.template_to_json(load_file(TEST_TEMPLATE_1))
        response = cloudformation.validate_template(TemplateBody=template)
        self.assertEqual(response['ResponseMetadata']['HTTPStatusCode'], 200)
    def test_validate_invalid_json_template_should_fail(self):