Add raw_request() to RestClient

TokenClientJSON and V3TokenClientJSON call self.http_obj.request()
directly for skipping an authentication request. However, the method
is internal in RestClient and we need to avoid the call for moving
RestClient to tempest-lib.
This patch adds raw_request() to RestClient for avoiding it.

Change-Id: I240b12139fd91747cf4ed96e0294a0bf082bcf93
3 files changed