Merge matrix feature branch into master.

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.

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+# Overview
+
+This directory contains scripts that facilitate building and running gRPC tests for combinations of language/runtimes (known as matrix).
+
+The setup builds gRPC docker images for each language/runtime and upload it to Google Container Registry (GCR). These images, encapsulating gRPC stack
+from specific releases/tag, are used to test version compatiblity between gRPC release versions.
+
+## Instructions for creating GCR images
+- Edit  `./client_matrix.py` to include desired gRPC release.
+- Run `tools/interop_matrix/create_matrix_images.py`.  Useful options:
+  - `--git_checkout` enables git checkout grpc release branch/tag.
+  - `--release` specifies a git release tag.  Make sure it is a valid tag in the grpc github rep.
+  - `--language` specifies a language.
+  For examle, To build all languages for all gRPC releases across all runtimes, do `tools/interop_matrix/create_matrix_images.py --git_checkout --release=all`.
+- Verify the newly created docker images are uploaded to GCR.  For example:
+  - `gcloud beta container images list --repository gcr.io/grpc-testing` shows image repos.
+  - `gcloud beta container images list-tags gcr.io/grpc-testing/grpc_interop_go1.7` show tags for a image repo.
+
+## Instructions for adding new language/runtimes*
+- Create new `Dockerfile.template`, `build_interop.sh.template` for the language/runtime under `template/tools/dockerfile/`.
+- Run `tools/buildgen/generate_projects.sh` to create corresponding files under `tools/dockerfile/`.
+- Add language/runtimes to `client_matrix.py` following existing language/runtimes examples.
+- Run `tools/interop_matrix/create_matrix_images.py` which will build and upload images to GCR.  Unless you are also building images for a gRPC release, make sure not to set `--gcr_tag` (the default tag 'master' is used for testing).
+
+*: Please delete your docker images at https://pantheon.corp.google.com/gcr/images/grpc-testing?project=grpc-testing afterwards.  Permissions to access GrpcTesting project is required for this step.
+
+## Instructions for creating new test cases
+- Create test cases by running `LANG=<lang> [RELEASE=<release>] ./create_testcases.sh`.  For example,
+  - `LANG=go ./create_testcases.sh` will generate `./testcases/go__master`, which is also a functional bash script.
+  - `LANG=go KEEP_IMAGE=1 ./create_testcases.sh` will generate `./testcases/go__master` and keep the local docker image so it can be invoked simply via `./testcases/go__master`.  Note: remove local docker images manually afterwards with `docker rmi <image_id>`.
+- Stage and commit the generated test case file `./testcases/<lang>__<release>`.
+
+## Instructions for running test cases against a GCR image
+- Run test cases by specifying `docker_image` variable inline with the test case script created above.
+For example:
+  - `docker_image=gcr.io/grpc-testing/grpc_interop_go1.7:master ./testcases/go__master` will run go__master test cases against `go1.7` with gRPC release `master` docker image in GCR.
+
+
+Note:
+- File path starting with `tools/` or `template/` are relative to the grpc repo root dir.  File path starting with `./` are relative to current directory (`tools/interop_matrix`).