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"
                },
...
```
try:
            return LogGroup_get_cfn_attribute_orig(self, attribute_name)
        except Exception:
            if attribute_name == 'Arn':
                return aws_stack.log_group_arn(self.name)
            raise
    LogGroup_get_cfn_attribute_orig = getattr(cw_models.LogGroup, 'get_cfn_attribute', None)
    cw_models.LogGroup.get_cfn_attribute = LogGroup_get_cfn_attribute
    
    def Lambda_Function_get_cfn_attribute(self, attribute_name):
        try:
            if attribute_name == 'Arn':
                return self.function_arn
            return Lambda_Function_get_cfn_attribute_orig(self, attribute_name)
        except Exception:
            if attribute_name in ('Name', 'FunctionName'):
                return self.function_name
            raise
    Lambda_Function_get_cfn_attribute_orig = lambda_models.LambdaFunction.get_cfn_attribute
    lambda_models.LambdaFunction.get_cfn_attribute = Lambda_Function_get_cfn_attribute
    
    def DynamoDB_Table_get_cfn_attribute(self, attribute_name):