demo: use ubuntu image instead of centos

support for machine deployment for centos 7+ has been discontinued for
kubeone

Signed-off-by: Deepika Upadhyay <[email protected]>

Author: Deepika Upadhyay

kubernetes-cluster-demo/terraform/terraform.tfvars.example

@@ -1,8 +1,8 @@
 cluster_name = "koor-demo"
-ssh_public_key_file = "~/.ssh/id_rsa.pub" // new ssh key
-control_plane_vm_count=3
-initial_machinedeployment_replicas=3
+ssh_public_key_file = "~/.ssh/id_ed25519.pub" // new ssh key
+control_plane_vm_count=2
+initial_machinedeployment_replicas=1
 worker_type="cpx41"
 control_plane_type="cpx31"
-os="centos"
-worker_os="centos"
+os="ubuntu"
+worker_os="ubuntu"