Set fixed number of workers for File-based IOITs

Autoscaling is a feature of google cloud dataflow runner that adds/removes worker nodes dynamically as the job runs. It can behave in a different way creating different test (runtime) results in consequent runs. In integration tests (such as IOIT but others also apply) we don't need such nondeterminism and it's best to have a fixed number of workers for every test execution.

IOITs use autoscaling but they shouldn't. This issue was created to disable it and set a fixed number of workers.

Side note: autoscaling is already disabled in Nexmark and load tests of core operations.