config: reset num_osds

config: reset num_osds

When collocating OSDs with other daemon, `num_osds` is incorrectly calculated
because `ceph-config` is called multiple times.

Indeed, the following code:
```
num_osds: "{{ lvm_list.stdout | default('{}') | from_json | length | int + num_osds | default(0) | int }}"
```

makes `num_osds` be incremented each time `ceph-config` is called.

We have to reset it in order to get the correct number of expected OSDs.

Signed-off-by: Guillaume Abrioux <gabrioux@redhat.com>