Add retries to apt cache update

Updating the apt cache sometimes fails due to it being
an operation that uses mirrors on the internet. The
task should be retried to reduce the chance of a
transient failure causing the playbook to fail.

Change-Id: I60d17d3cae97aed3d9bfd26652ef6306a678399c
Partial-Bug: #1750656

tasks/memcached_install-apt.yml

@@ -25,6 +25,10 @@
   apt:
     update_cache: yes
   when: "ansible_date_time.epoch|float - apt_cache_stat.stat.mtime > {{cache_timeout}}"
+  register: update_apt_cache
+  until: update_apt_cache | success
+  retries: 5
+  delay: 2
 
 - name: Install apt packages
   apt: