Fix OS selection in CI - Do not fallback to Ubuntu if unknown value

What does this MR do and why?

Related reference(s)

Test coverage

CI configuration

CI pipelines perform an update for both management and workload clusters, this update will NOT perform a ClusterAPI rolling update (deletion and creation of new K8s nodes) by default.

For some cases, it may be relevant to perform more complex tests.

Theses features can be activated in an MR by adding one of these labels to the MR and will apply to the next pipelines.

• adding the label ci-featuretest-rolling-update pipelines will perform a node rolling update in the -update jobs (without version upgrades)
• adding the label ci-featuretest-upgrade-from-1.2.1 pipelines will perform an upgrade from Sylva 1.2.1 to your dev branch (including a k8s version upgrade resulting in a node rolling update)