fix #2670: cloudinit enable SLAAC

fix #2670: cloudinit enable SLAAC

cloud-init's SLAAC option was disabled in 2018 because there was no
support for it. Now that cloud-init 19.4 or newer versions are more
widespread, we can finally reenable it.

Also include minimum required cloud-init version for SLAAC support in
format description.

Tested on Ubuntu 20.04 (ci 20.4), CentOS 8 (ci 19.4), Debian 10 (ci
20.2).

Signed-off-by: Mira Limbeck <m.limbeck@proxmox.com>