undercloud: add dropbear-initramfs to restic_prune hosts

Adds the ability to remotely decrypt your restic prune hosts.

You could setup wireguard-initramfs too with https://gitlab.phowork.fr/phowork/iac/ansible/roles/wireguard-initramfs.git (and usage example) but I think dropbear-initramfs should be enough in your current setup.