split nginx configuration file

* add a new variable nginx_log_dir, the base directory for storing logs
  for nginx servers
* make sure this directory exists
* make nginx read {{nginx_conf_dir}}/conf.d for extra configuration
  parameters
* make nginx read {{nginx_conf_dir}}/sites-enabled for server
  configurations
* extract a server configuration that can be used as a template for
  creating new server configurations (or for checking that basic
  installation of nginx works)
* make nginx by default to use all available cores
specific nginx templates

proxy
  reverse-proxy setup

static
  static web site setup
wip