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"
                },
...
```
stack_name = self._get_param('StackName')
        backend = self.cloudformation_backend
        stack = backend.get_stack(stack_name)
        if not stack:
            
            stack = ([stk for id, stk in backend.deleted_stacks.items() if stk.name == stack_name] or [0])[0]
        if not stack:
            raise ValidationError(stack_name,
                message='Unable to find stack "%s" in region %s' % (stack_name, aws_stack.get_region()))
        template = self.response_template(responses.DESCRIBE_STACK_EVENTS_RESPONSE)
        return template.render(stack=stack)
    responses.CloudFormationResponse.describe_stack_events = cf_describe_stack_events
    
    for region in boto3.session.Session().get_available_regions('lambda'):
        if region not in lambda_models.lambda_backends:
            lambda_models.lambda_backends[region] = lambda_models.LambdaBackend(region)
    
    def initialize_resources(self):
        def set_status(status):
            self._add_stack_event(status)
            self.status = status
        self.resource_map.create()
        self.output_map.create()
        def run_loop(*args):