Right now the --concurrency flag for stestr run takes in an integer, if that value is 0 it runs nproc workers, if that value is greater than 0 it runs that many workers. But we don't have a way to specify a value that is a fraction of nproc easily (like nproc/2 or nproc/4). This can be useful for some test suites, especially if they're very resource intensive, where running nproc workers isn't via