Implement pipeline step to create Kubernetes cluster via gardener

[OliverNocon]

Enhancement proposal: Implement a step to create kubernetes "shoot" cluster via gardener.

• Use gardener project service user and store the credentials in Jenkins
• Use the shoot yaml templates as provided here: https://github.com/gardener/gardener/tree/master/example -> 90_* (different templates per hyperscale provider
• run kubectl with the template to create the cluster (or use a Kubernetes deployment step???)
• poll if the cluster is ready (use kubctl in a do...while loop)
• get the kubeconfig from the new created cluster (kubectl get secrets...)
• store the clusters kubeconfig (kubectl get secrets .kubeconfig -o jsonpath="{.data.kubeconfig}" | base64 -D > kubeconfig.yaml )
• use the new kubeconfig in a Kubernetes deployment step to do a deployment
• optional: step to delete the cluster

[OliverNocon]

will close this for now, let's see if we see demands at some point in time