add two new roles

supervisor

* add supervisor package
* config it
* enable and start supervisor daemon
* the role has a parameter -- services -- a list of supervisor services
  to be created: see roles/supervisor/templates/service.conf.j2 for
  parameter list (probably should go to a overall doc of sort)

supervisor-service

* to be documented
start creation of nginx role

* install the package
* enable the service
* make sure it's running
* copy nginx.conf file to templates/ directory
* render the template
* add a handler to reload nginx in case its configurations files are
  changed
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
finalise nginx role

* add two templates:
  proxy
    reverse-proxy setup

  static
    static web site setup
* add tasks to
  * create corresponding log directory
  * create corresponding server configuration file
wip
wip2
wip3