Add rkt gc task (#2945)

roles/rkt/files/rkt-gc.sh

+#!/bin/bash
+rkt gc

roles/rkt/tasks/main.yml

 ---
-
 - name: Install rkt
   import_tasks: install.yml
   when: not ansible_os_family in ["CoreOS", "Container Linux by CoreOS"]
+
+- name: Set up cron job to do garbage cleanup
+  copy:
+    src: rkt-gc.sh
+    dest: /etc/cron.hourly/rkt-gc.sh
+    owner: root
+    group: root
+    mode: 0750
+  when: not ansible_os_family in ["CoreOS", "Container Linux by CoreOS"]