CAPO: automatically push OS image to Glance via sylva-units

Today, pushing the OS image in OpenStack Glance is a manual step done prior to starting Sylva installation tools. This prevents us from having something clean and automated, driven from sylva-core, to consume OS images produced by diskimage-builder in our CAPO CI (see #527 (closed)).

We could create the Glance image as part of capo-cluster-resources as a first improvement.

Doing that would however have the drawback of taking time and using bw for each CI job.

Another idea would be to have a smarter mechanism, able to fetch the image checksum from the diskimage-builder OCI artifact (same format as the one found in Glance, see openstack image show 1723948f-ed47-4f7c-851c-aec49ce6cfeb -f value -c checksum) and see if some openstack image already exists with the same checksum, and push an image to Glance only if this is the case.