osd: include osd_objectstore in metadata reported to mon

Provide visibility into the OSD backend.

Signed-off-by: Sage Weil <sage@inktank.com>
Merge pull request #1815 from ceph/osd-metadata

osd: include osd_objectstore in metadata reported to mon

Reviewed-by: Loic Dachary <loic@dachary.org>