ceph_objectstore_tool: --op list now prints [pg,object]

The format of the output of --op list was changed to include the PG to
which the object belong. It simplifies the loop in
ceph_objectstore_tool.py.

http://tracker.ceph.com/issues/10376 Fixes: #10376

Signed-off-by: Loic Dachary <ldachary@redhat.com>