Fix Tempest tests failing on gate-solum-devstack-dsvm

Due to a new commit on tempest, all our devstack tests were
failing. This was due to an inheritance we have to RestClient
from tempest. The signature of the constructor has changed.

This patch changes the signature of our inherited classes to
match the change from tempest

Fixes bug 1275697

Change-Id: I848d4b70d3b5129295d4aec5edbd4c03428ca0cc