Add ignore_errors to fix minor bug with fallback source for apt-key.

Add ignore_errors to execute 'add galera apt-keys using fallback
source'.
Bug was found and described by David Wilde.

Change-Id: I22f68fd78161498d636972f3181031da093daf7d
Closes-Bug: 1594467
Co-Authored-By: David Wilde <david.wilde@rackspace.com>
(cherry picked from commit b268c29)

tasks/galera_client_pre_install.yml

@@ -31,6 +31,7 @@
   until: add_keys|success
   retries: 5
   delay: 2
+  ignore_errors: true
   tags:
     - galera-client-apt-keys