Robust maas-rack readiness probe

With the existing readiness probe mechanism, if log rotation occurs
then it may lead maas rack pod to show false not ready. Instead save
the success message of rack registration to a file and then use it in
the readiness probe.

Change-Id: I569b99186d398db44a10824dc3fe8c745b13a4ac

charts/maas/templates/bin/_register-rack-controller.sh.tpl

@@ -20,6 +20,7 @@ register_maas_rack() {
     if maas-rack register --url="${MAAS_ENDPOINT}" --secret="${MAAS_REGION_SECRET}";
     then
         echo "Successfully registered with MaaS Region Controller"
+        touch '/tmp/maas-rack.done'
         break
     else
         echo "Unable to register with ${MAAS_ENDPOINT}... will try again"

charts/maas/templates/statefulset-rack.yaml

@@ -99,9 +99,8 @@ spec:
             timeoutSeconds: 60
             exec:
               command:
-                - /bin/bash
-                - -c
-                - journalctl -u register-rack-controller -n 5 | grep -q 'Successfully registered with MaaS Region Controller'
+                - ls
+                - /tmp/maas-rack.done
           volumeMounts:
             - mountPath: /sys/fs/cgroup
               name: host-sys-fs-cgroup