SW: add cloud-init to GRML_FULL

cloud-init is useful in cloud environments, also Proxmox provides support
for it.

This adds 11.5 MB of additional disk space. We consider adding it to
GRML_SMALL as well (adding 14MB there). But for the time being verify its
usefulness before adding it to grml-small as well.

Thanks Joel Wirāmu Pauling for suggesting this.

See https://github.com/grml/grml/issues/124