mgr/cephadm: make osd create on an existing LV idempotent

mgr/cephadm: make osd create on an existing LV idempotent

If we try to prepare an LV that was already prepared, ceph-volume will
return an error message and code.  We want our osd create command to be
idempotent, though, so recognize the error string and continue.

This is an ugly hack, but quicker than changing ceph-volume behavior, and
it is sufficient to stop all of the teuthology failures.

The second part of this is that we have to deploy the daemon on OSDs that
are already prepared and already exist in our osdmap beforehand, but have
never started.

Works-around: https://tracker.ceph.com/issues/44313
Signed-off-by: Sage Weil <sage@redhat.com>