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')
        value = b''
        if bucket_name != BUCKET_MARKER_LOCAL:
            value = s3_client.get_object(Bucket=bucket_name, Key=key_name)['Body'].read()
        return s3_models.FakeKey(name=key_name, value=value)
    s3_models.S3Backend.get_key = get_key
    
    def clean_json(resource_json, resources_map):