crimson: use given osd_fsid when mkfs

crimson: use given osd_fsid when mkfs

MemStore does not have this feature, but CyanStore needs it. because
`qa/tasks/ceph.py` uses following steps when creating/starting an OSD

1. ceph-osd --mkfs --mkkey -i <osd_id> --monmap <monmap>
2. read the osd_fsid of osd.<osd_id>
3. ceph osd new <osd_fsid> <osd_id>

when we mkfs for the OSD, the osd_fsid is still unknown. so we cannot
use the configured one, as it is always empty. in that case, we need to
use a random uuid, and persist it to both ${osd_data}/fsid and
superblock.osd_fsid

Signed-off-by: Kefu Chai <kchai@redhat.com>