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"
                },
...
```
props = cloudformation_json['Properties']
        name = props['Name']
        region_name = props.get('Region') or aws_stack.get_region()
        description = props.get('Description') or ''
        id = props.get('Id') or short_uid()
        return apigw_models.RestAPI(id, region_name, name, description)
    def RestAPI_get_cfn_attribute(self, attribute_name):
        if attribute_name == 'Id':
            return self.id
        if attribute_name == 'Region':
            return self.region_name
        if attribute_name == 'Name':
            return self.name
        if attribute_name == 'Description':
            return self.description
        if attribute_name == 'RootResourceId':
            for id, resource in self.resources.items():
                if resource.parent_id is None:
                    return resource.id
            return None
        raise UnformattedGetAttTemplateException()
    @classmethod
    def Deployment_create_from_cloudformation_json(cls, resource_name, cloudformation_json, region_name):