rolling_update: refact set_fact `mon_host`

rolling_update: refact set_fact `mon_host`

each monitor node should select another monitor which isn't itself.
Otherwise, one node in the monitor group won't set this fact and causes
failure.

Typical error:
```
TASK [create potentially missing keys (rbd and rbd-mirror) when mon is containerized] ***
task path: /home/jenkins-build/build/workspace/ceph-ansible-prs-dev-update_docker_cluster/rolling_update.yml:200
Thursday 22 November 2018  14:02:30 +0000 (0:00:07.493)       0:02:50.005 *****
fatal: [mon1]: FAILED! => {}

MSG:

The task includes an option with an undefined variable. The error was: 'dict object' has no attribute u'mon2'
```

Signed-off-by: Guillaume Abrioux <gabrioux@redhat.com>
(cherry picked from commit af78173584f1b3a99515e9b94f450be22420c545)