mon: fix force peer addition task

mon: fix force peer addition task

when using `monitor_interface`, if nodes don't have same interface names
this task will fail like following:

```
fatal: [argo010]: FAILED! => {
    "msg": "The task includes an option with an undefined variable. The error was: 'ansible.vars.hostvars.HostVarsVars object' has no attribute u'ansible_enp1s0f0'\n\nThe error appears to have been in '/usr/share/ceph-ansible/roles/ceph-mon/tasks/docker/main.yml': line 19, column 3, but may\nbe elsewhere in the file depending on the exact syntax problem.\n\nThe offending line appears to be:\n\n\n- name: ipv4 - force peer addition as potential bootstrap peer for cluster bringup - monitor_interface\n  ^ here\n"
}

```

Closes: https://bugzilla.redhat.com/show_bug.cgi?id=1876551
Signed-off-by: Guillaume Abrioux <gabrioux@redhat.com>