objectstore_tool: filter --op list and explore all PGs

The positional object name is used to filter the output of --op list and
only show the objects with a matching name. If both the object name and
the pgid are omitted, all objects from all PGs are displayed.

The output format is changed from

    {"oid":"GROUP","key":"","snapid":-2,
     "hash":2659194943,"max":0,"pool":0,"namespace":""}

to

    [["0.7_head",{"oid":"GROUP","key":"","snapid":-2,
                  "hash":2659194943,"max":0,"pool":0,
                  "namespace":""}]]

where the first member is the pgid where the object is found.

Signed-off-by: Loic Dachary <ldachary@redhat.com>
objectstore_tool: parse new object description format

The object format changed from

    {json object}

to

    [pgid,{json object}]

The parser is updated accordingly. If the --pgid is present, check that
it equals the pgid from the object description.

Signed-off-by: Loic Dachary <ldachary@redhat.com>
objectstore_tool: test --op list variants

Signed-off-by: Loic Dachary <ldachary@redhat.com>