update: fix undefined error when no mgr group is declared
update: fix undefined error when no mgr group is declared

if mgr group isn't defined in inventory, that task will fail with
undefined error.

Signed-off-by: Guillaume Abrioux <30a9df5a0293a6108675377fa1673f7aa713f915@redhat.com>

ceph/ceph-ansible,ceph/ceph-ansible