Enable automatic reboot after unattended-upgrades

...so that we don't need to manually reboot the machine after critical
security updates are applied.

Install the `update-notifier-common` package as it's required for the
automatic reboots to work. `update-notifier-common` is responsible for
creating the `/var/run/reboot-required` file.

If a reboot is required, it will happen immediately after
`unattended-upgrades` has run at approximately 06:25 in the morning.
Disable automatic reboots for transition-logs

Automatic reboots on a transition-logs node may interrupt log
processing. Continue to manage reboots for these machines manually.

@jennyd explains why automatic reboots may cause an issue for
`transition-logs`:

> If the logs [processing
> script](https://github.com/alphagov/ci-puppet/blob/17af91244d793410e940a0781841a31b27cdd99c/modules/ci_environment/files/process_transition_logs.sh)
> is interrupted between starting to [process
> logs](https://github.com/alphagov/ci-puppet/blob/master/modules/ci_environment/files/process_transition_logs.sh#L38)
> and [committing the
> results](https://github.com/alphagov/ci-puppet/blob/master/modules/ci_environment/files/process_transition_logs.sh#L51)
> then it will [exit
> early](https://github.com/alphagov/ci-puppet/blob/master/modules/ci_environment/files/process_transition_logs.sh#L20-L23)
> on the next run, and won't be able to process any more logs until that
> is cleared up.

> In addition, if a logfile is only partially processed, we will end up
> with some duplicated results from it (and inflated hit counts getting
> into Transition) because our processing [appends to a cache
> file](https://github.com/alphagov/pre-transition-stats/blob/abe386e43c82fb08c6f1b8775deca8025e40d006/lib/transition/ingest/awk/common-log.awk#L102)
> which covers a whole day. At that stage it would be very hard to
> identify which lines in the cache file came from the partially
> processed logfile. Since our logfiles are rotated hourly, the possible
> impact of that is limited, but it's not ideal.
Merge pull request #241 from alphagov/73607174-auto-reboot

[#73607174] Enable automatic reboot after unattended-upgrades