Switch using token_client to the latest for gate

On tempest-lib 0.8.0, TokenClientJSON is marked as deprecated
and a new TokenClient became standard instead.
In Tempest unit tests, we are stubbing TokenClientJSON but it
doesn't work at all now.
This patch makes these unit tests switch to TokenClient for
passing the gate job.

Change-Id: I6404fd8215a619a90eaeea22afa479639d150872
3 files changed