Merge pull request #5971 from sreecha/stress_test_flexible_config

templates/tools/dockerfile/gcp_api_libraries.include

...ls/dockerfile/grpc_interop_stress_cxx/Dockerfile.template

tools/dockerfile/grpc_interop_stress_cxx/Dockerfile

...ockerfile/grpc_interop_stress_cxx/build_interop_stress.sh

tools/jenkins/build_interop_stress_image.sh

tools/run_tests/stress_test/README.md

tools/run_tests/stress_test/configs/opt-tsan-asan.json

tools/run_tests/stress_test/configs/opt.json

tools/run_tests/stress_test/run_on_gke.py

tools/run_tests/stress_test/run_stress_tests_on_gke.py