Changed python proto build to use shared protos

src/python/.gitignore

src/python/grpcio/commands.py

src/python/grpcio/tests/interop/_insecure_interop_test.py

src/python/grpcio/tests/interop/_secure_interop_test.py

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

src/python/grpcio/tests/interop/empty.proto

src/python/grpcio/tests/interop/messages.proto

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

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

src/python/grpcio/tests/interop/test.proto

tools/run_tests/run_interop_tests.py

tools/run_tests/run_tests.py