update: reset mon_host after mons upgrade

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>