purge-container: fix wrong syntax

we want a default value for `mon_group_name`, not for
`groups[mon_group_name]`.

Signed-off-by: Guillaume Abrioux <gabrioux@redhat.com>
(cherry picked from commit d0b3cb7f85)

infrastructure-playbooks/purge-docker-cluster.yml

@@ -308,19 +308,18 @@
   gather_facts: true
   become: true
 
-  pre_tasks:
-  - name: gather monitors facts
-    setup:
-    delegate_to: "{{ item }}"
-    delegate_facts: True
-    with_items: "{{ groups[mon_group_name] | default('mons') }}"
-
   tasks:
 
   - import_role:
       name: ceph-defaults
       private: false
 
+  - name: gather monitors facts
+    setup:
+    delegate_to: "{{ item }}"
+    delegate_facts: True
+    with_items: "{{ groups.get(mon_group_name, 'mons') }}"
+
   - name: get all the running osds
     shell: |
       systemctl list-units | grep 'loaded[[:space:]]\+active' | grep -oE "ceph-osd@([0-9]{1,2}|[a-z]+).service"