build: test using @grpc/grpc-js in CI (googleapis#604)

* build: test using @grpc/grpc-js in CI

* simplify configs

Author: jkwlui

handwritten/storage/.kokoro/continuous/node8/system-test-grpcjs.cfg

@@ -0,0 +1,12 @@
+# Download resources for system tests (service account key, etc.)
+gfile_resources: "/bigstore/cloud-devrel-kokoro-resources/google-cloud-nodejs"
+
+env_vars: {
+    key: "TRAMPOLINE_BUILD_FILE"
+    value: "github/nodejs-storage/.kokoro/system-test.sh"
+}
+
+env_vars: {
+    key: "GOOGLE_CLOUD_USE_GRPC_JS"
+    value: "1"
+}

handwritten/storage/.kokoro/presubmit/node8/system-test-grpcjs.cfg

@@ -0,0 +1,12 @@
+# Download resources for system tests (service account key, etc.)
+gfile_resources: "/bigstore/cloud-devrel-kokoro-resources/google-cloud-nodejs"
+
+env_vars: {
+    key: "TRAMPOLINE_BUILD_FILE"
+    value: "github/nodejs-storage/.kokoro/system-test.sh"
+}
+
+env_vars: {
+    key: "GOOGLE_CLOUD_USE_GRPC_JS"
+    value: "1"
+}