Add enablement for cloud-init in stretch
Add enablement for cloud-init in stretch

 * Enable cloud-init:
   - Add gnupg early so next apt-get update does not fail
   - Add Scaleway PPA as /etc/apt/sources.list/scaleway.list in overlay
   - Install cloud-init and dmidecode
   - Disable systemd units now taken care of by cloud-init
   - Override cloud-init root disablement to keep backward compatibility

scaleway/image-debian