Drop the legacy and un-used _interface
Drop the legacy and un-used _interface

We used to run JSON and XML tests. XML has been dropped now,
but still in the code there are remains of the logic to switch
between JSON and XML. Cleaning them up.

Partially-implements: bp/resource-cleanup

Change-Id: I39ecfbd99861136a6a2adc3ce56bfdbba96d5c9b

openstack/sahara-tests,openstack/sahara-tests,openstack/sahara-scenario,openstack/sahara-scenario,openstack/sahara-tests,openstack/sahara-scenario,openstack/sahara-tests