Split the apt cache update when repos change from the install task

When the repositories change, the apt cache needs updating to bring
in the new indexes. Recently this has failed quite often. In the
hope that we can get more diagnostic information, and in order to
make the tasks a litte easier to read, we split them into two.

Change-Id: I4b1bb0d3c318a1f07b4a1d055faed65691565320

tasks/galera_install_apt.yml

@@ -84,12 +84,22 @@
     backup: yes
   changed_when: false
 
+- name: Update Apt cache
+  apt:
+    update_cache: yes
+  when:
+    - add_galera_repo | changed or add_percona_repo | changed
+  register: update_apt_cache
+  until: update_apt_cache | success
+  retries: 5
+  delay: 2
+
 - name: Install galera_server role remote packages (apt)
   apt:
     name: "{{ galera_packages_list }}"
     state: "{{ galera_server_package_state }}"
     update_cache: yes
-    cache_valid_time: "{{ (add_galera_repo | changed or add_percona_repo | changed) | ternary('0', cache_timeout) }}"
+    cache_valid_time: "{{ cache_timeout }}"
   register: install_remote_apt_packages
   until: install_remote_apt_packages|success
   retries: 5