Migrate distrib, interop, and stress to GA API

...thon/grpcio_tests/tests/interop/_insecure_interop_test.py

...python/grpcio_tests/tests/interop/_secure_interop_test.py

src/python/grpcio_tests/tests/interop/client.py

src/python/grpcio_tests/tests/interop/methods.py

src/python/grpcio_tests/tests/interop/server.py

src/python/grpcio_tests/tests/stress/client.py

src/python/grpcio_tests/tests/stress/metrics_server.py

test/distrib/python/distribtest.py