Add 'owncloud' role

- this role installs and configures ownCloud (http://owncloud.org/);

- ownCloud by default uses nginx, php5 and mysql roles to configure the
  environment on local host, you can disable that and configure your own
  services;

- by default ownCloud automatically finishes installation with admin
  username 'admin-$USER' and 'password' password, or random if role
  'secret' is used, you can disable that behaviour using a variable;

- you can use either MySQL (default), PostgreSQL or SQLite as the
  database backend;