Migrated python performance tests to use GA API

src/python/grpcio_tests/tests/qps/benchmark_client.py

src/python/grpcio_tests/tests/qps/benchmark_server.py

src/python/grpcio_tests/tests/qps/qps_worker.py

src/python/grpcio_tests/tests/qps/worker_server.py

src/python/grpcio_tests/tests/unit/test_common.py

tools/run_tests/performance/run_worker_python.sh

tools/run_tests/performance/scenario_config.py