fixed possible race condition problem with Mutex.synchronize
removed methods for parallel script execution from pool_manager, fixed bug in delete_vm method: ensures that vm is stopped before the pool manager tries to delete the vm including its storage, refactored vm list generation, added test case for occupy-release-workflow
implemented multithread handler/ script runner, so test scripts can be executed in parallel using the full potential of the vm pool, added test case for script which uses a dummy pool manager (white box test)