purge: gather monitors facts in OSD purge

the OSD part of the purge delegates commands on monitor node, we need to
gather monitors facts to know the `ansible_hostname` fact that is used
in the `docker_exec_cmd` fact.

Signed-off-by: Guillaume Abrioux <gabrioux@redhat.com>
(cherry picked from commit 1a4a6ec855)

infrastructure-playbooks/purge-docker-cluster.yml

@@ -308,6 +308,13 @@
   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: