Build openSUSE-based kubeadm images

We need openSUSE + kubeadm OS images, as in https://gitlab.com/sylva-projects/sylva-elements/diskimage-builder/-/releases/0.1.0 we only have openSUSE + rke2, to allow for addressing the left part of sylva-projects/sylva-core#727 (closed) (rke2 images and CI jobs are covered by sylva-projects/sylva-core!1262 (merged)).