Add support for skeleton directory (useradd -k) in user module

New option in users module to use a custom skeleton

For example

    - name: ensure vhost users are present
      user:
        name: {{ item['user'] }}
         home: /var/www/{{ item['host'] }}
         skeleton: /etc/wwwskel
         password: {{ item['password'] }}
         shell: {{ item['shell'] }}
       with_items: vhosts