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')
        stack = self.cloudformation_backend.get_stack(stack_name)
        logical_resource_id = self._get_param('LogicalResourceId')
        if not stack:
            msg = ('Unable to find CloudFormation stack "%s" in region %s' %
                   (stack_name, aws_stack.get_region()))
            if aws_stack.get_region() != self.region:
                msg = '%s/%s' % (msg, self.region)
            LOG.warning(msg)
            response = aws_responses.flask_error_response(msg, code=404, error_type='ResourceNotFoundException')
            return 404, response.headers, response.data
        for stack_resource in stack.stack_resources:
            
            
            if stack_resource and stack_resource.logical_resource_id == logical_resource_id:
                resource = stack_resource
                break
        else:
            raise ValidationError(logical_resource_id)
        template = self.response_template(
            responses.DESCRIBE_STACK_RESOURCE_RESPONSE_TEMPLATE)
        return template.render(stack=stack, resource=resource)
    responses.CloudFormationResponse.describe_stack_resource = describe_stack_resource
    
    def cf_describe_stack_events(self):