Use shell for apt-get update

The apt ansible module requires python-apt, but this command is
trying to update the apt cache which is, by definition, out of
date since we just changed the sources list.   Just use the shell
so we don't require anything extra on images.

Change-Id: I41313af70193f97e2ef433d0ac0a0d274fafddb7

roles/configure-mirrors/tasks/mirror.yaml

@@ -22,6 +22,5 @@
 # Make sure OS does not have a stale package cache.
 - name: Update apt cache
   become: yes
-  apt:
-    update_cache: yes
+  command: apt-get update
   when: ansible_os_family == 'Debian'