Since openjdk8 is dead to us (see #19113), we cannot leave openjdk8 in
the list of runtimes in client_matrix.py. The list of runtimes is now
the defaults to use, which includes master, and you can specify
alternative runtimes per-release.
Fixes#19113
Features included in this merge:
- Added script to build and upload docker image for matrix test.
- Added script to create test cases and created go__master testcases based on it.
- Created dictionary for runtimes and gRPC releases for supported languages.
- Added go 1.7 and 1.8 Dockerfile/templates.
See tools/interop_matrix/README.md for details.