Add base::archive class to Hiera

Given that we use Hiera as a lightweight ENC here, we need to add this class
to Hiera in order for things to Just Work(tm).
Add `archive` class

This class creates a command, to be run by the `govuk-backup` user, in order
to remove back-ups created more than 30 days ago.

This works fine:

vagrant@obnode0:~$ sudo crontab -u govuk-backup -l
0 9 * * * find /dev/mapper/backup-data/* -mtime +30 -exec rm {} /\;
Amend command

This commit amends the command run by cron to ensure only files are ever
matched, and replace`--exec rm } \\;` with `--delete`, as it's slightly
easier to read.