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