objectstore_tool: tests only needs 1 machine

Signed-off-by: Loic Dachary <ldachary@redhat.com>
objectstore_tool: keep all json object representation

For erasure coded pools to be tested, the json object representation
must be preserved for all PG because they are all different. The
internal representation is changed from

   db[name]["pgid"] = pg
   db[name]["json"] = objjson

to a per pg map:

   db[name].setdefault("pg2json", {})[pg] = objjson

and the rest of the code is modified to adapt accordingly.

Signed-off-by: Loic Dachary <ldachary@redhat.com>
objectstore_tool: add hinfo_key tests for erasure coded objects

When an hinfo_key attribute is found, assume an erasure coded object and
verify set-attr/get-attr works as expected by removing its content and
restoring it.

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