move dockerfiles where they belong

tools/dockerfile/grpc_interop_csharp/Dockerfile

tools/dockerfile/grpc_interop_csharp/build_interop.sh

tools/dockerfile/grpc_interop_cxx/Dockerfile

tools/dockerfile/grpc_interop_cxx/build_interop.sh

tools/dockerfile/grpc_interop_go/Dockerfile

tools/dockerfile/grpc_interop_go/build_interop.sh

tools/dockerfile/grpc_interop_http2/Dockerfile

tools/dockerfile/grpc_interop_http2/build_interop.sh

tools/dockerfile/grpc_interop_java/Dockerfile

tools/dockerfile/grpc_interop_java/build_interop.sh

tools/dockerfile/grpc_interop_node/Dockerfile

tools/dockerfile/grpc_interop_node/build_interop.sh

tools/dockerfile/grpc_interop_php/Dockerfile

tools/dockerfile/grpc_interop_php/build_interop.sh

tools/dockerfile/grpc_interop_python/Dockerfile

tools/dockerfile/grpc_interop_python/build_interop.sh

tools/dockerfile/grpc_interop_ruby/Dockerfile

tools/dockerfile/grpc_interop_ruby/build_interop.sh

tools/dockerfile/grpc_interop_stress_cxx/Dockerfile

...ockerfile/grpc_interop_stress_cxx/build_interop_stress.sh