Bring Python interop tests to the beta API

src/python/grpcio_health_checking/commands.py

src/python/grpcio_test/.gitignore

src/python/grpcio_test/commands.py

...python/grpcio_test/grpc_interop/_insecure_interop_test.py

src/python/grpcio_test/grpc_interop/_secure_interop_test.py

src/python/grpcio_test/grpc_interop/client.py

src/python/grpcio_test/grpc_interop/empty.proto

src/python/grpcio_test/grpc_interop/empty_pb2.py

src/python/grpcio_test/grpc_interop/messages.proto

src/python/grpcio_test/grpc_interop/messages_pb2.py

src/python/grpcio_test/grpc_interop/methods.py

src/python/grpcio_test/grpc_interop/server.py

src/python/grpcio_test/grpc_interop/test.proto

src/python/grpcio_test/grpc_interop/test_pb2.py

src/python/grpcio_test/grpc_protoc_plugin/test.proto

src/python/grpcio_test/setup.py