purge: do not fail on purge ceph files

On systems running docker there is an issue with lxfs that results in
the find command returning 1 but actually did the job.
e.g: on a system with docker runnning find /var will give us the
following error:

find:
'/var/lib/lxcfs/cgroup/devices/lxc/x1/system.slice/systemd-update-utmp.service/devices.deny':
Permission denied
find:
'/var/lib/lxcfs/cgroup/devices/lxc/x1/system.slice/dev-random.mount/devices.allow':
Permission denied
...
...

However ceph files got deleted so we ignore the error.

Signed-off-by: Sébastien Han <seb@redhat.com>

infrastructure-playbooks/purge-cluster.yml

@@ -593,6 +593,7 @@
 
   - name: remove upstart and apt logs and cache
     shell: "find /var -name '*ceph*' -delete"
+    failed_when: false
     when: ansible_distribution == 'Ubuntu'
 
   - name: request data removal