tests: fix `_get_osd_id_from_host()` in TestOSDs()

tests: fix `_get_osd_id_from_host()` in TestOSDs()

We must initialize `children` variable in `_get_osd_id_from_host()`,
otherwise, if for any reason the deployment has failed and result with
an osd host with no OSD registered, we won't enter in the condition,
therefore, `children` is never set and the function tries to return
something undefined.

Typical error:
```
E       UnboundLocalError: local variable 'children' referenced before assignment
```

Fixes: #2860
Signed-off-by: Guillaume Abrioux <gabrioux@redhat.com>
(cherry picked from commit 9a65ec231d7f76655caa964e1d9228ba7a910dea)