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.

Note that the schedule for reboots and related cron jobs is as follows
(all times in UTC):

  - [Transition CDN logs][cdn_logs] are backed up at 22:10
  - the [asset backup jobs][asset_backup] happen at 04:13 and 04:20
  - unattended-upgrades on this box runs (and potentially could cause an
    immediate reboot) at 06:25 (as defined by Ubuntu's cron.daily)
  - the offsite backups from the main GOV.UK stack occur at 08:13
  - the [Transition upload logs][upload_logs] occur between 00:00 and
    08:59 (depending on the FQDN)

It's the last backup that we care about most and reboots should be
infrequent; I don't foresee any major conflicts as the assets should
back up relatively quickly (they are using `librsync` deltas provided by
[duplicity]).

Tested in Vagrant using:

```bash
vagrant@obnode0:/etc/apt/apt.conf.d$ sudo touch /var/run/reboot-required
vagrant@obnode0:/etc/apt/apt.conf.d$ sudo unattended-upgrade

Broadcast message from root@obnode0
        (unknown) at 15:35 ...

The system is going down for reboot NOW!
vagrant@obnode0:/etc/apt/apt.conf.d$ Connection to 127.0.0.1 closed by remote host.
Connection to 127.0.0.1 closed.
```

[asset_backup]: https://github.gds/gds/puppet/blob/fa551c6e88bc7416e481960919031c2cca7806a4/modules/backup/manifests/assets.pp#L61-L71
[cdn_logs]: https://github.com/alphagov/ci-puppet/blob/969ac59016ef41261572f1862e8c542e06f3b6af/modules/cdn_logs/manifests/backup.pp#L11-L12
[duplicity]: http://duplicity.nongnu.org/
[offsite_backup]: https://github.gds/gds/puppet/blob/a74e16da55bbcb2737957edad44718d5841bd896/modules/backup/manifests/offsite.pp#L30-L31
[upload_logs]: https://github.com/alphagov/ci-puppet/blob/969ac59016ef41261572f1862e8c542e06f3b6af/modules/ci_environment/manifests/transition_logs/remote_users.pp#L24-L26