* 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.
KubernetesVersion sent by the init command now controls
all downloaded binaries, if they depend on the k8s version.
* Move all download links into /internal/versions.
* Unify files in /internal/versions package
* Move image download links into VersionConfigs
and thus make them dependant on the k8s version,
where the image version is specific to the k8s version.
* Don't specify patch version in k8sVersion
