Nginx 'user' option fix

* NGINX: fixed issue with user www-data option in nginx.conf.erb, which caused on Centos

  Original issue description: https://tickets.opscode.com/browse/COOK-3397?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel
  
  We can't use standart:
  ```
   use www-data; # in bz-webserver/templates/default/nginx.conf.erb
  ```
  because value of this setting can be different for different platforms
  https://github.com/miketheman/nginx/blob/master/attributes/default.rb#L35

* Updated 'ubuntu' cookbook from "1.1.7" to "1.1.8"