Switch using token_client to the recommended

On tempest-lib 0.8.0, TokenClientJSON is marked as deprecated
and a new TokenClient became standard instead.
Now tempest.log contains the warning message like
"TokenClientJSON class was deprecated and renamed to TokenClient".

This patch makes Tempest use TokenClient.

Change-Id: I37f513dfcc79fb31941f7fb2697867b4f490cfc6
1 file changed