backup-and-restore: fix check on 'target_node' variable

backup-and-restore: fix check on 'target_node' variable

If the user doesn't pass a valid name (present in the inventory)
the playbook will fail like following:

```
fatal: [localhost -> {{ target_node }}]: FAILED! =>
  msg: |-
    The task includes an option with an undefined variable. The error was: "hostvars['10.70.46.40']" is undefined
```

Closes: https://bugzilla.redhat.com/show_bug.cgi?id=2051640
Signed-off-by: Guillaume Abrioux <gabrioux@redhat.com>