rolling_update: fix mgr start with mon collocation

cec994b introduced a regression when a mgr is collocated with a mon.
During the mon upgrade, the mgr service is masked to avoid to be
restarted on packages update.
Then the start mgr task is failing because the service is still masked.
Instead we should unmask it.

Fixes: #5983

Signed-off-by: Dimitri Savineau <dsavinea@redhat.com>

infrastructure-playbooks/rolling_update.yml

@@ -221,6 +221,7 @@
         name: ceph-mgr@{{ ansible_hostname }}
         state: started
         enabled: yes
+        masked: no
       when: inventory_hostname in groups[mgr_group_name] | default([])
             or groups[mgr_group_name] | default([]) | length == 0