Optionally enable unattended-upgrades' reboots when required

Sometimes after an upgrades it's required to reboot the machine. E.g. after a Kernel upgrade. unattended-upgrades supports that.

Introduce a variable: automatic_reboot with possible values yes and no, the latter (no) being the default.

When set to yes, or not specified at all, make it default to no, meaning, the automatic reboot feature of unattended-upgrades will not be used.