Separate NegativeRestClient from rest_client

We have a plan that both RestClient and NegativeRestClient are moved
to tempest-lib, and RestClient will be moved before NegativeRestClient.
This patch separates NegativeRestClient from rest_client for moving
these classes smoothly.

Change-Id: I4ffa85fb0eefa96a71f1b279947caa6028632251
4 files changed