QoS: Fix shutdown problem in StageTest

Was a problem in the tearDownClass of StageTest. Explicit shutdown of OSDs
DIR was missing. Also remove the no superfluous NullStage and NullStageTest.
server: benchmark: Fix problem with default DIRs in BenchmarkConfig
server: benchmark: Controller: rethrow exceptions from benchmark tasks
QoS: Adapt BenchmarkRunner and BenchmarkStage to new BenchmarkConfig