AB#2074: Choosable K8S Version (#277)

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

bootstrapper/internal/kubernetes/k8sapi/resources/cluster_autoscaler.go

@@ -1,6 +1,7 @@
 package resources
 
 import (
+	"github.com/edgelesssys/constellation/internal/versions"
 	"google.golang.org/protobuf/proto"
 	apps "k8s.io/api/apps/v1"
 	k8s "k8s.io/api/core/v1"
@@ -434,7 +435,7 @@ func NewDefaultAutoscalerDeployment(extraVolumes []k8s.Volume, extraVolumeMounts
 						Containers: []k8s.Container{
 							{
 								Name:            "cluster-autoscaler",
-								Image:           clusterAutoscalerImage,
+								Image:           versions.ClusterAutoscalerImage,
 								ImagePullPolicy: k8s.PullIfNotPresent,
 								LivenessProbe: &k8s.Probe{
 									ProbeHandler: k8s.ProbeHandler{