ceph-osd: Fix start_osds.yml in check mode

ceph-osd: Fix start_osds.yml in check mode

This construct doesn't work as intended since ansible/ansible#74212:

```
ceph_osd_ids.stdout | default('{}') | from_json
```

That PR made the `command` module return `stdout` even in check mode (setting
it to the empty string), so `default()` has no effect in that case and
`from_json()` fails to parse an empty string.

Instead, `default()` needs to be invoked with its second argument set to
`True`, so that it replaces any `False` value (such as an empty string) with
its first argument:

```
ceph_osd_ids.stdout | default('{}', True) | from_json
```

Signed-off-by: Benoît Knecht <bknecht@protonmail.ch>