mgr: append balancer module to ceph_mgr_modules

otherwise the osd play in rolling_update can fail when it tries to
disable it before upgrading osd nodes.

Signed-off-by: Guillaume Abrioux <gabrioux@redhat.com>

roles/ceph-mgr/tasks/common.yml

@@ -92,6 +92,6 @@
     ceph_mgr_modules: "{{ ceph_mgr_modules | union(['dashboard', 'prometheus']) }}"
   when: dashboard_enabled | bool
 
-- name: append pg_autoscaler module to ceph_mgr_modules
+- name: append balancer, pg_autoscaler module to ceph_mgr_modules
   set_fact:
-    ceph_mgr_modules: "{{ ceph_mgr_modules | union(['pg_autoscaler']) }}"
+    ceph_mgr_modules: "{{ ceph_mgr_modules | union(['pg_autoscaler', 'balancer']) }}"