schema: add cloud-init-username attribute
The username used by cloud images which use cloud-init changes depending on the distro. A source with some of them is https://docs.openstack.org/image-guide/obtain-images.html.
I've added this property to the oldest release with the 'cloud-init' attribute for each distro, hoping that we can use inheritance to get it for the newer releases, since it is unlikely to change that much.
Signed-off-by: Rafael Fonseca r4f4rfs@gmail.com