Show delegate_to hint in callback output

This pull request adds additional output to `on_unreachable`, `on_failed`, `on_ok`, `on_error` and `on_skipped` to output a hint indicating the host specified in `delegate_to` (which also covers `local_action`).

Sample output:

```
$ ansible-playbook -i srv1.example.org, delegate.yml

PLAY [all] ********************************************************************

TASK: [delegate_to localhost] *************************************************
ok: [srv1.example.org -> localhost] => {
    "msg": "foo"
}

TASK: [local_action] **********************************************************
ok: [srv1.example.org -> 127.0.0.1] => {
    "msg": "foo"
}

PLAY RECAP ********************************************************************
srv1.example.org            : ok=2    changed=0    unreachable=0    failed=0
```