Follow up Maintenance mode except office IP address.

If all the pages return 503 in maintenance mode, AWS ELB will mark your instance fail, so end up your customer cannot see the maintenance page...

We need to set the "ping" page always available, so that ELB won't remove our instance.

# AWS ELB Ping?
if ($uri = "/path/to/ping") {  
  set $maintenance "${maintenance}P";