Add script to add user

[lvps]

Create a script to add a user and its SSH key on boulangerie, start from /root/bin/mkuser (which doesn't set permissions correctly) and the procedure on the wiki

[andrea-artuso]

Should we create also a script for managing users on rocco? For now it will only add their SSH key to auth users, but in the future we could make a dedicated user for everyone, so it would be easier to manage everything also with specific permissions.