install-kubernetes: enable minikube version pinning

This change enables using a particular version of minikube for
debugging purpose.

Change-Id: Ide264165f12185e8313a16b6b6286ab74b32aa15

roles/install-kubernetes/README.rst

@@ -6,3 +6,8 @@ An ansible role to install kubernetes.
    :default: True
 
    If true, installs a Minikube cluster.
+
+.. zuul:rolevar:: minikube_version
+   :default: latest
+
+   The version of Minikube to install.

roles/install-kubernetes/defaults/main.yaml

@@ -1 +1,2 @@
 install_kubernetes_with_cluster: True
+minikube_version: latest

roles/install-kubernetes/tasks/minikube.yaml

@@ -5,7 +5,7 @@
 
 - name: Download Minikube
   get_url:
-    url: https://storage.googleapis.com/minikube/releases/latest/minikube-linux-amd64
+    url: https://storage.googleapis.com/minikube/releases/{{ minikube_version }}/minikube-linux-amd64
     dest: /tmp/minikube
     mode: 0755
   when: stat_result.stat.exists == False