Finish work to remove cast to schedule_run_instance

This blueprint proposes that the work be completed to remove the
schedulers schedule_run_instance method and compute managers
run_instance method from the spawn instance execution path.

Part of blueprint remove-cast-to-schedule-run-instance

Change-Id: I777d0a7c9ba9a92391e2aa5c68c7418e8e2b8072
Merge "Finish work to remove cast to schedule_run_instance"