Add fastcgi_param SCRIPT_FILENAME

This could be missing depending on Nginx package versions in Ubuntu so
we're adding it to ensure it's always there.

roles/nginx/templates/wordpress.conf.j2

@@ -5,5 +5,6 @@ location / {
 location ~ \.php$ {
   try_files $uri =404;
   include fastcgi_params;
+  fastcgi_param SCRIPT_FILENAME $document_root$fastcgi_script_name;
   fastcgi_pass unix:/var/run/php5-fpm-$site_name.sock;
 }