[#73607174] Enable automatic reboots for security updates

__DEPENDS ON https://github.gds/gds/govuk_mirror-deployment/pull/24__

Tell `unattended-upgrades(1)` to automatically reboot the mirror
servers if required. Currently `unattended-upgrades` is only configured
to apply secutiy updates. `unattended-upgrades(1)` is run by
`cron.daily`.

Time time when `cron.daily` is run can be specified for each provider
(by setting the hour) and each node (by setting the minute) so that we
can address any issues caused by rebooting before the next mirror or set
of mirrors is rebooted, thus ensuring that one set of mirrors and one
node in each set of mirrors is always available.

Note that this will also purge any cron resources that have been created
manually.

I also repurposed the `node.*.yaml` specified in the Hiera hiearchy so
to `hostname`, which enabled me to specify Hiera data specific to
each node (`mirror0` or `mirror1`). We're not currently using
`node.*.yaml`.

* * *

Also, fix the Puppet deprecation warning about the use of `import`.