osd: create osd id if it does not exist

osd: create osd id if it does not exist

Most tools create the osd id before trying to start ceph-osd.  Notably,
teuthology does not.  We could fix that, but we would be changing behavior,
as the osd boot will happily create the osd id on the fly for us.  Other
provisioning tools might rely on that behavior.  Instead, just allocate
the id sooner in the process (if necessary).

Signed-off-by: Sage Weil <sage@redhat.com>