Add missing 'nginx_log_dir' setting to rubber-passenger_nginx.yml

#320 added a 'nginx_log_dir' setting to specify location of the Nginx log directory, but didn't add the new setting to all related yml files. I looked through the templates and I think that rubber-passenger_nginx.yml is the only other yml file effected.
Merge pull request #370 from willkoehler/nginx_log_dir_fix

Add missing 'nginx_log_dir' setting to rubber-passenger_nginx.yml