Goal: 

As a developer, I want run nova-scheduler with native threading, so that nova will work post Eventlet.

Acceptance Criteria:

• nova-scheduler works in threading mode
• nova-scheduler works in Eventlet mode
• nova-scheduler uses the proper oslo.service backend based on the mode
• at least one upstream CI job exists that runs tempest with nova-scheduler using threading mode
• as a special acceptance criteria here I want to only close this ticket when the upstream patches are landed and the nova-next job is running scheduler in threading mode to all incoming patches.