typical error:

```
2021-02-01 03:11:09,809 p=93834 u=cephuser n=ansible | TASK [ceph-rgw : check if the realm system user already exists] ***************************************************************************************************************************************************
2021-02-01 03:11:09,809 p=93834 u=cephuser n=ansible | Monday 01 February 2021  03:11:09 -0500 (0:00:00.084)       0:14:38.607 *******
2021-02-01 03:11:09,836 p=93834 u=cephuser n=ansible | fatal: [ceph-kvm-ms2-1611241931591-node7-rgw]: FAILED! =>
  msg: |-
    The task includes an option with an undefined variable. The error was: 'None' has no attribute 'realm'
```

This task should be skipped when `zone_users` is undefined.

Closes: https://bugzilla.redhat.com/show_bug.cgi?id=1922998



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