common: do not omit shard when ghobject NO_GEN is set

Do not silence the display of shard_id when generation is NO_GEN.
Erasure coded objects JSON representation used by ceph_objectstore_tool
need the shard_id to find the file containing the chunk.

Minimal testing is added to ceph_objectstore_tool.py

http://tracker.ceph.com/issues/10063 Fixes: #10063

Signed-off-by: Loic Dachary <ldachary@redhat.com>
(cherry picked from commit dcf09aed121f566221f539106d10283a09f15cf5)