crimson/osd: write the 'osd_key' meta on OSD::mkfs().

crimson/osd: write the 'osd_key' meta on OSD::mkfs().

This commit fixes an issue identified during the Rook-crimson effort.

Missing the `write_meta()` on `osd_key` made the CephX inoperational
because of imposibility to load the keyring. Disabling CephX in turn
caused the auth method negotation to fail when reaching out to a monitor.

```
ERROR 2021-09-28 21:19:46,598 [shard 0] none - auth: unable to find a keyring on /var/lib/ceph/osd/ceph-0/keyring: (2) No such file or directory
ERROR 2021-09-28 21:19:46,598 [shard 0] none - AuthRegistry(0x7fa38c322b68) no keyring found at /var/lib/ceph/osd/ceph-0/keyring, disabling cephx
...
INFO  2021-09-28 21:19:46,601 [shard 0] monc - get_auth_request(con=[client.?(temp_mon_client) 172.17.0.1:0/2910147961@63138 >> mon.? v2:10.108.187.31:3300/0], auth_method=0)
INFO  2021-09-28 21:19:46,601 [shard 0] monc - get_auth_request no methods is supported
...
WARN  2021-09-28 21:20:06,612 [shard 0] monc - cannot establish the active_con with any mon
```

Signed-off-by: Radoslaw Zarzynski <rzarzyns@redhat.com>