During the service client development, we have switched json import
to oslo_serialization on Tempest side.
token_client modules were migrated before that, and they still import
json module.
This patch switches to oslo_serialization for token_client modules.
In addition, an internal method _json_loads() of RestClient is not
used with the above change. So this patch removes _json_loads().
Change-Id: I4c731b2ce35c4fb85904cab89dbe943ae95b3b52
37a767c30c