update: reset mon_host after mons upgrade

after all mon are upgraded, let's reset mon_host which is used in the
rest of the playbook for setting `container_exec_cmd` so we are sure to
use the right value.

Typical error:

```
failed: [mds0 -> mon0] (item={u'path': u'/var/lib/ceph/bootstrap-mds/ceph.keyring', u'name': u'client.bootstrap-mds', u'copy_key': True}) => changed=true
  ansible_loop_var: item
  cmd:
  - docker
  - exec
  - ceph-mon-mon2
  - ceph
  - --cluster
  - ceph
  - auth
  - get
  - client.bootstrap-mds
  delta: '0:00:00.016294'
  end: '2019-09-27 13:54:58.828835'
  item:
    copy_key: true
    name: client.bootstrap-mds
    path: /var/lib/ceph/bootstrap-mds/ceph.keyring
  msg: non-zero return code
  rc: 1
  start: '2019-09-27 13:54:58.812541'
  stderr: 'Error response from daemon: No such container: ceph-mon-mon2'
  stderr_lines: <omitted>
  stdout: ''
  stdout_lines: <omitted>
```

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

infrastructure-playbooks/rolling_update.yml

@@ -264,6 +264,19 @@
       delay: "{{ health_mon_check_delay }}"
       when: containerized_deployment | bool
 
+
+- name: reset mon_host
+  hosts: "{{ mon_group_name|default('mons') }}"
+  become: True
+  tasks:
+    - import_role:
+        name: ceph-defaults
+
+    - name: reset mon_host fact
+      set_fact:
+        mon_host: "{{ hostvars[groups[mon_group_name][0]]['ansible_hostname'] }}"
+
+
 - name: upgrade ceph mgr nodes when implicitly collocated on monitors
   vars:
     health_mon_check_retries: 5