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['StageName']
        deployment_id = props.get('Id') or short_uid()
        description = props.get('Description') or ''
        return apigw_models.Deployment(deployment_id, name, description)
    @classmethod
    def Resource_create_from_cloudformation_json(cls, resource_name, cloudformation_json, region_name):
        props = cloudformation_json['Properties']
        region_name = props.get('Region') or aws_stack.get_region()
        path_part = props.get('PathPart')
        api_id = props.get('RestApiId')
        parent_id = props.get('ParentId')
        id = props.get('Id') or short_uid()
        return apigw_models.Resource(id, region_name, api_id, path_part, parent_id)
    @classmethod
    def Method_create_from_cloudformation_json(cls, resource_name, cloudformation_json, region_name):
        props = cloudformation_json['Properties']
        method_type = props.get('HttpMethod')
        authorization_type = props.get('AuthorizationType')
        return apigw_models.Method(method_type, authorization_type)
    apigw_models.RestAPI.create_from_cloudformation_json = RestAPI_create_from_cloudformation_json
    apigw_models.RestAPI.get_cfn_attribute = RestAPI_get_cfn_attribute
    apigw_models.Deployment.create_from_cloudformation_json = Deployment_create_from_cloudformation_json
    apigw_models.Resource.create_from_cloudformation_json = Resource_create_from_cloudformation_json
    apigw_models.Method.create_from_cloudformation_json = Method_create_from_cloudformation_json
    
    
    def describe_stack_resource(self):