Retry perform a synced flush task while upgrading elasticsearch

The synced flush fails due to concurrent indexing operations.
The HTTP status code in that case will be 409 CONFLICT. We can
retry this task until returns success.

Change-Id: I57f9a009b12715eed8dfcf829a71f418d2ce437b

ansible/roles/elasticsearch/tasks/upgrade.yml

@@ -21,6 +21,10 @@
     body_format: json
   delegate_to: "{{ groups['elasticsearch'][0] }}"
   run_once: true
+  retries: 10
+  delay: 5
+  register: result
+  until: ('status' in result) and result.status == 200
 
 # Stop all elasticsearch containers before applying configuration to ensure
 # handlers are triggered to restart them.