purge-cluster: add a check for a device list

People who use `osd_auto_discovery` feature will not have a devices
variable populated, so we first check for the existence of devices.

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

purge-cluster.yml

@@ -59,6 +59,13 @@
      state: absent
 
   tasks:
+  - name: check for a device list
+    fail:
+      msg: "OSD automatic discovery was detected, purge cluster does not support this scenario. If you want to purge the cluster, manually provide the list of devices in group_vars/osds using the devices variable."
+    when:
+      devices is not defined and
+      osd_auto_discovery
+
   - name: get osd numbers
     shell: "if [ -d /var/lib/ceph/osd ] ; then ls /var/lib/ceph/osd | cut -d '-' -f 2 ; fi"
     register: osd_ids