Merge pull request #3605 from jtattermusch/interop_improvements

tools/jenkins/build_interop_image.sh

tools/jenkins/grpc_interop_csharp/Dockerfile

tools/jenkins/grpc_interop_csharp/build_interop.sh

tools/jenkins/grpc_interop_cxx/Dockerfile

tools/jenkins/grpc_interop_cxx/build_interop.sh

tools/jenkins/grpc_interop_java/Dockerfile

tools/jenkins/grpc_interop_java/build_interop.sh

tools/jenkins/grpc_interop_node/Dockerfile

tools/jenkins/grpc_interop_node/build_interop.sh

tools/jenkins/grpc_interop_php/Dockerfile

tools/jenkins/grpc_interop_php/build_interop.sh

tools/jenkins/grpc_interop_ruby/Dockerfile

tools/jenkins/grpc_interop_ruby/build_interop.sh

tools/run_tests/dockerjob.py

tools/run_tests/jobset.py

tools/run_tests/run_interop_tests.py