HealthCheck Middleware

Glance has imported healchcheck middleware preferred in https://blueprints.launchpad.net/glance/+spec/healthcheck-middleware. It's helpful for nova to get thi function.
HealthCheck Middleware provides <SERVER>:<PORT>/healtcheck endpoint. This endpoint behaves similar way as it does in Swift. Normal operations returns 200 OK and if config option disable_path is enabled and that path exists it will return 503 DISABLED BY FILE.