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"
                },
...
```
result = SNS_Topic_get_cfn_attribute(self, attribute_name)
        if attribute_name.lower() in ['arn', 'topicarn']:
            result = aws_stack.fix_account_id_in_arns(result)
        return result
    IAM_Role_get_cfn_attribute_orig = iam_models.Role.get_cfn_attribute
    iam_models.Role.get_cfn_attribute = IAM_Role_get_cfn_attribute
    
    @classmethod
    def Lambda_create_from_cloudformation_json(cls, resource_name, cloudformation_json, region_name):
        resource_name = cloudformation_json.get('Properties', {}).get('FunctionName') or resource_name
        return Lambda_create_from_cloudformation_json_orig(resource_name, cloudformation_json, region_name)
    Lambda_create_from_cloudformation_json_orig = lambda_models.LambdaFunction.create_from_cloudformation_json
    lambda_models.LambdaFunction.create_from_cloudformation_json = Lambda_create_from_cloudformation_json
    
    @classmethod
    def Mapping_create_from_cloudformation_json(cls, resource_name, cloudformation_json, region_name):
        props = cloudformation_json.get('Properties', {})
        func_name = props.get('FunctionName') or ''
        if ':lambda:' in func_name:
            props['FunctionName'] = aws_stack.lambda_function_name(func_name)
        return Mapping_create_from_cloudformation_json_orig(resource_name, cloudformation_json, region_name)
    Mapping_create_from_cloudformation_json_orig = lambda_models.EventSourceMapping.create_from_cloudformation_json
    lambda_models.EventSourceMapping.create_from_cloudformation_json = Mapping_create_from_cloudformation_json
    
    @classmethod
    def Lambda_update_from_cloudformation_json(cls,