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

New PR for #5677.

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