Revert "cleanup: don't update_cache when adding a new repo"

This reverts commit d8e1f4d83c
because the tasks are implemented that way for the following reason.

If apt_repository fails to update the apt cache after updating the
configuration, retries don't register there was a change and so no
attempt is made to update the cache by the module on the second attempt.
This failure can result in a failure to install packages.

This change adds an apt module task to update the cache if the
apt_repository task registers a change. This means updating the cache
will get retried on failure and no longer fail silently.

This was all explained in the commit message for
I41de2b9a98977bb89de812a9fbc85a9f99d62942 but no notes were
added to the tasks, resulting in the confusion. To prevent this happening
again, we add the comments from Id059dbec3466cb1ef3ea567249f52384a8ade515
into these tasks.

Change-Id: I110f6fa3b6c6341ec4a8bd8cf69ae61bbbb50689

tasks/galera_client_install_apt.yml

@@ -47,10 +47,30 @@
   tags:
     - galera-client-apt-keys
 
+# When updating the cache in the apt_repository
+# task, and the update fails, a retry does not
+# detect a change the second attempt and therefore
+# does not update the cache, resulting in a changed
+# repo config, but no updated cache. To work around
+# this bug we implement the change of repo config
+# and the cache update as two seperate tasks.
 - name: Add galera repo
   apt_repository:
     repo: "{{ galera_client_repo }}"
     filename: "{{ mariadb_repo_filename }}"
     state: present
+    update_cache: no
+  register: add_repo
+  tags:
+    - galera-client-repos
+
+- name: Update Apt cache
+  apt:
+    update_cache: yes
+  when: add_repo is changed
+  register: update_apt_cache
+  until: update_apt_cache is success
+  retries: 5
+  delay: 2
   tags:
     - galera-client-repos