Merge pull request #4674 from xfxyjwf/bazel

Add kokoro configs for bazel build.

kokoro/linux/bazel/build.sh

+#!/bin/bash
+#
+# Build file to set up and run tests
+
+# Change to repo root
+cd $(dirname $0)/../../..
+
+git submodule update --init --recursive
+bazel test :protobuf_test

kokoro/linux/bazel/continuous.cfg

+# Config file for running tests in Kokoro
+
+# Location of the build script in repository
+build_file: "protobuf/kokoro/linux/bazel/build.sh"
+timeout_mins: 15

kokoro/linux/bazel/presubmit.cfg

+# Config file for running tests in Kokoro
+
+# Location of the build script in repository
+build_file: "protobuf/kokoro/linux/bazel/build.sh"
+timeout_mins: 15