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"
                },
...
```
cloudformation = aws_stack.connect_to_service('cloudformation')
    response = cloudformation.describe_stack_resources(StackName=stack_name)
    for resource in response['StackResources']:
        if resource['LogicalResourceId'] == resource_logical_id:
            return resource
def list_stack_resources(stack_name):
    cloudformation = aws_stack.connect_to_service('cloudformation')
    response = cloudformation.list_stack_resources(StackName=stack_name)
    return response['StackResourceSummaries']
def get_queue_urls():
    sqs = aws_stack.connect_to_service('sqs')
    response = sqs.list_queues()
    return response['QueueUrls']
def get_topic_arns():
    sqs = aws_stack.connect_to_service('sns')
    response = sqs.list_topics()
    return [t['TopicArn'] for t in response['Topics']]
class CloudFormationTest(unittest.TestCase):
    def test_create_delete_stack(self):
        cloudformation = aws_stack.connect_to_resource('cloudformation')
        cf_client = aws_stack.connect_to_service('cloudformation')
        s3 = aws_stack.connect_to_service('s3')
        sns = aws_stack.connect_to_service('sns')
        apigateway = aws_stack.connect_to_service('apigateway')
        template = template_deployer.template_to_json(load_file(TEST_TEMPLATE_1))
        
        stack_name = 'stack-%s' % short_uid()
        cloudformation.create_stack(StackName=stack_name, TemplateBody=template)
        
        def check_stack():