osd: discover osd_objectstore on the fly

osd: discover osd_objectstore on the fly

Applying and passing the OSD_BLUESTORE/FILESTORE on the fly is wrong for
existing clusters as their config will be changed.

Typically, if an OSD was prepared with ceph-disk on filestore and we
change the default objectstore to bluestore, the activation will fail.
The flag osd_objectstore should only be used for the preparation, not
activation. The activate in this case detects the osd objecstore which
prevents failures like the one described above.

Signed-off-by: Sébastien Han <seb@redhat.com>
(cherry picked from commit 4c5113019893c92c4d75c9fc457b04158b86398b)