Fix vagrant custom instance name prefix

Vagrantfile

@@ -100,11 +100,11 @@ Vagrant.configure("2") do |config|
           #ansible.tags = ['download']
           ansible.groups = {
             # The first three nodes should be etcd servers
-            "etcd" => ["k8s-0[1:3]"],
+            "etcd" => ["#{$instance_name_prefix}-0[1:3]"],
             # The first two nodes should be masters
-            "kube-master" => ["k8s-0[1:2]"],
+            "kube-master" => ["#{$instance_name_prefix}-0[1:2]"],
             # all nodes should be kube nodes
-            "kube-node" => ["k8s-0[1:#{$num_instances}]"],
+            "kube-node" => ["#{$instance_name_prefix}-0[1:#{$num_instances}]"],
             "k8s-cluster:children" => ["kube-master", "kube-node"],
           }
         end