repair bucket instance metadata

1. identify bucket instance id for bucket named "testbucket"

~/ceph/build $ bin/radosgw-admin metadata get bucket:testbucket | grep bucket_id
            "bucket_id": "4b022f22-54ca-49e5-b547-9e8225d98de1.4159.1",

2. read bucket instance metadata into testbucket.orig.json

~/ceph/build $ bin/radosgw-admin metadata get bucket.instance:testbucket:4b022f22-54ca-49e5-b547-9e8225d98de1.4159.1 > testbucket.orig.json

3. replace "num_shards": 0 with 1

~/ceph/build $ sed 's/"num_shards": 0/"num_shards": 1/' testbucket.orig.json > testbucket.new.json

4. write updated bucket instance metadata from testbucket.new.json

~/ceph/build $ bin/radosgw-admin metadata put bucket.instance:testbucket:4b022f22-54ca-49e5-b547-9e8225d98de1.4159.1 < testbucket.new.json