* Update image version
* Introduce 'ValidK8sVersion' type. Ensures that consumers
of the k8sVersion receive a valid version, without
having to do their own validation.
* Add testcase to check that kubeadm accepts the currently provided
version.
AB#2074: Add configurable k8s version
Configurable version flow:
* cli config holds/validates k8sVersion
* InitCluster receive a k8sVersion arg
* InitCluster creates CM "k8s-version"
* kubeadm's InitConfiguration receives k8sVersion
* joinservice spec mounts/reads k8s-version CM
* joinservice supplies k8sVersion via JoinTicketResponse
Other changes:
* Remove unused test code (FakeK8SClient)
* move VersionConfig map to /internal/versions
* installk8sComponents is now a function instead of a method
