Rename rebooter to "final_touch"

Some changes have been done:
  * it's possible to specify which command to execute on the minion
  * the program exits with a meaningful message if it's not run as root
    user
Fixed error inside of final_touch state
Handle also sshd
Better handling of the 'amministratore' user
Ensure amministratore is a memeber of the admin group

Otherwise amministratore won't be able to execute sudo.
Handle groups on the server

Ensure the admin group is present.
Fix minor issues
Refactoring of software pillar

Software pillar refactored to offer specif list of software to the
different types of machines we support.
Refactoring of lightdm autologin

Log automatically as 'studente' on the lab machines, use 'insegnante' on the
other ones.
Added desktop configuration file

Ensure firefox is installed and its link is shown on the desktop
Added 'insegnante' to the user list

Fixed also a small issue with the 'amministratore' user.
Introduce new machine type: pc-aula
Add user 'insegnante' only on 'pc-aula*'