upgrade: fix standby_mdss group creation

This commit fixes the standby_mdss group creation by using `{{ item }}`.

Signed-off-by: Guillaume Abrioux <gabrioux@redhat.com>
(cherry picked from commit c4fc8cc878)

infrastructure-playbooks/rolling_update.yml

@@ -577,7 +577,7 @@
 
     - name: create standby_mdss group
       add_host:
-        name: "{{ standby_mdss }}"
+        name: "{{ item }}"
         groups: standby_mdss
         ansible_host: "{{ hostvars[item]['ansible_host'] | default(omit) }}"
         ansible_port: "{{ hostvars[item]['ansible_port'] | default(omit) }}"