This reverts commit 6c4687e4.

The version of curl in Ubuntu 20.04 does not have the --retry-all-errors
option yet. It will be available when we move on to Ubuntu 22.04.

Also use the nice VARIABLE instead of hardcoding it.

7 retries takes around 2 minutes with curl exponential delays.

Without the full name, systemctl start assumed it to be a service name.

This will help troubleshooting, as the future sysadmins will connect to
the machine, run `journalctl -f ` to see what's wrong with the machine,
and in about 30 seconds they will see:

  Error fetching machine configuration!
  Error fetching machine info!
  If this is a new machine, did you add it into labmanager-admin?

So they will know what's wrong, and how to correct it.

These are not debug messages, they should be always on to see what's happening
on the machine.

The first commit (the one that was reverted) contained a spurious,
unrelated change.

This reverts commit 2ed8cbd1.

The bta-no-usbpen was a link to /bin/sleep.

Passaggio ai target secondari (esame e graphical) attraverso comando 'start' invece di 'isolate' (NetworkManager non ripartiva più dopo il passaggio di target)

New feature (fix for someone...): don't erase user's home if the computer crashes or reboots during the exam.
The user's home is erased (if required) only when the exam mode starts the first time.

Fix: if the web service does not reply on machine boot, try 5 times then switch to graphical target.
If the WS does not reply when the machine is already in some exam mode or normal, do nothing.

Runtime parameters change had to depend on esame-get-config-boot service instead.
FIX2: firefox home set at runtime now kill every firefox instance before update firefox config file.

Fixed get-config to work on machines that were turned off in exam mode and must wake up in normal mode.