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 = aws_stack.connect_to_service('s3')
        cloudformation = aws_stack.connect_to_service('cloudformation')
        
        s3.create_bucket(Bucket=TEST_ARTIFACTS_BUCKET, ACL='public-read')
        s3.put_object(Bucket=TEST_ARTIFACTS_BUCKET, Key=TEST_ARTIFACTS_PATH, Body=TEST_TEMPLATE_5)
        
        buckets_before = len(s3.list_buckets()['Buckets'])
        stack_name = 'stack-%s' % short_uid()
        cloudformation.create_stack(StackName=stack_name, TemplateBody=TEST_TEMPLATE_6)
        
        buckets_after = len(s3.list_buckets()['Buckets'])
        self.assertEqual(buckets_after, buckets_before + 1)