Add NGiNX 'server_tokens off' directive.

http://wiki.nginx.org/HttpCoreModule#server_tokens
http://techtalk.virendrachandak.com/how-to-hide-nginx-version-number-in-headers-and-errors-pages/#axzz2qOKJnSrM
http://www.cyberciti.biz/tips/linux-unix-bsd-nginx-webserver-security.html

Whether to send the Nginx version number in error pages and Server header.

Context: http,server,location, so set in http.