move interop test docker files to a subdir

tools/dockerfile/interoptest/grpc_interop_csharp/Dockerfile

...ckerfile/interoptest/grpc_interop_csharp/build_interop.sh

tools/dockerfile/interoptest/grpc_interop_cxx/Dockerfile

.../dockerfile/interoptest/grpc_interop_cxx/build_interop.sh

tools/dockerfile/interoptest/grpc_interop_go/Dockerfile

...s/dockerfile/interoptest/grpc_interop_go/build_interop.sh

tools/dockerfile/interoptest/grpc_interop_http2/Dockerfile

...ockerfile/interoptest/grpc_interop_http2/build_interop.sh

tools/dockerfile/interoptest/grpc_interop_java/Dockerfile

...dockerfile/interoptest/grpc_interop_java/build_interop.sh

tools/dockerfile/interoptest/grpc_interop_node/Dockerfile

...dockerfile/interoptest/grpc_interop_node/build_interop.sh

tools/dockerfile/interoptest/grpc_interop_php/Dockerfile

.../dockerfile/interoptest/grpc_interop_php/build_interop.sh

tools/dockerfile/interoptest/grpc_interop_python/Dockerfile

...ckerfile/interoptest/grpc_interop_python/build_interop.sh

tools/dockerfile/interoptest/grpc_interop_ruby/Dockerfile

...dockerfile/interoptest/grpc_interop_ruby/build_interop.sh