Refactored 'nginx' role

- new default values for many variables, allow for smaller nginx server definitions
- added support for php5-fpm role and PHP5 upstream configuration
- added support for status pages, both for nginx (/nginx_status) and php5-fpm,
  configurable either by server or globally
- added configurable "server_name default;" option; you can select which nginx server
  imports configuration from /etc/nginx/http-default.d/ directory