build: update release configuration

This PR was generated using Autosynth. 🌈

Here's the log from Synthtool:
```
synthtool > Executing /tmpfs/src/git/autosynth/working_repo/synth.py.
synthtool > Ensuring dependencies.
synthtool > Pulling artman image.
latest: Pulling from googleapis/artman
Digest: sha256:f3d61ae45abaeefb6be5f228cda22732c2f1b00fb687c79c4bd4f2c42bb1e1a7
Status: Image is up to date for googleapis/artman:latest
synthtool > Cloning googleapis.
synthtool > Running generator for google/datastore/artman_datastore.yaml.
synthtool > Generated code into /home/kbuilder/.cache/synthtool/googleapis/artman-genfiles/js/datastore-v1.
synthtool > Replaced '../../package.json' in src/v1/datastore_client.js.
.eslintignore
.eslintrc.yml
.github/ISSUE_TEMPLATE/bug_report.md
.github/ISSUE_TEMPLATE/feature_request.md
.github/ISSUE_TEMPLATE/support_request.md
.jsdoc.js
.kokoro/common.cfg
.kokoro/continuous/node10/common.cfg
.kokoro/continuous/node10/test.cfg
.kokoro/continuous/node11/common.cfg
.kokoro/continuous/node11/test.cfg
.kokoro/continuous/node6/common.cfg
.kokoro/continuous/node6/test.cfg
.kokoro/continuous/node8/common.cfg
.kokoro/continuous/node8/docs.cfg
.kokoro/continuous/node8/lint.cfg
.kokoro/continuous/node8/samples-test.cfg
.kokoro/continuous/node8/system-test-grpcjs.cfg
.kokoro/continuous/node8/system-test.cfg
.kokoro/continuous/node8/test.cfg
.kokoro/docs.sh
.kokoro/lint.sh
.kokoro/presubmit/node10/common.cfg
.kokoro/presubmit/node10/test.cfg
.kokoro/presubmit/node11/common.cfg
.kokoro/presubmit/node11/test.cfg
.kokoro/presubmit/node6/common.cfg
.kokoro/presubmit/node6/test.cfg
.kokoro/presubmit/node8/common.cfg
.kokoro/presubmit/node8/docs.cfg
.kokoro/presubmit/node8/lint.cfg
.kokoro/presubmit/node8/samples-test.cfg
.kokoro/presubmit/node8/system-test-grpcjs.cfg
.kokoro/presubmit/node8/system-test.cfg
.kokoro/presubmit/node8/test.cfg
.kokoro/presubmit/windows/common.cfg
.kokoro/presubmit/windows/test.cfg
.kokoro/publish.sh
.kokoro/release/publish.cfg
.kokoro/samples-test.sh
.kokoro/system-test.sh
.kokoro/test.bat
.kokoro/test.sh
.kokoro/trampoline.sh
.nycrc
.prettierignore
.prettierrc
CODE_OF_CONDUCT.md
CONTRIBUTING.md
LICENSE
codecov.yaml
renovate.json

> grpc@1.19.0 install /tmpfs/src/git/autosynth/working_repo/node_modules/grpc
> node-pre-gyp install --fallback-to-build --library=static_library

node-pre-gyp WARN Using needle for node-pre-gyp https download 
[grpc] Success: "/tmpfs/src/git/autosynth/working_repo/node_modules/grpc/src/node/extension_binary/node-v57-linux-x64-glibc/grpc_node.node" is installed via remote

> protobufjs@6.8.8 postinstall /tmpfs/src/git/autosynth/working_repo/node_modules/protobufjs
> node scripts/postinstall


> @google-cloud/datastore@3.1.1 prepare /tmpfs/src/git/autosynth/working_repo
> npm run compile


> @google-cloud/datastore@3.1.1 compile /tmpfs/src/git/autosynth/working_repo
> tsc -p . && cp -r src/v1 build/src && cp -r proto* build && cp test/*.js build/test

npm notice created a lockfile as package-lock.json. You should commit this file.
npm WARN optional SKIPPING OPTIONAL DEPENDENCY: fsevents@1.2.7 (node_modules/fsevents):
npm WARN notsup SKIPPING OPTIONAL DEPENDENCY: Unsupported platform for fsevents@1.2.7: wanted {"os":"darwin","arch":"any"} (current: {"os":"linux","arch":"x64"})

added 1146 packages from 1337 contributors and audited 7192 packages in 30.676s
found 1 low severity vulnerability
  run `npm audit fix` to fix them, or `npm audit` for details

> @google-cloud/datastore@3.1.1 fix /tmpfs/src/git/autosynth/working_repo
> gts fix && eslint '**/*.js' --fix


/tmpfs/src/git/autosynth/working_repo/samples/concepts.js
  25:29  error  "@google-cloud/datastore" is not found  node/no-missing-require

/tmpfs/src/git/autosynth/working_repo/samples/quickstart.js
  20:29  error  "@google-cloud/datastore" is not found  node/no-missing-require

/tmpfs/src/git/autosynth/working_repo/samples/system-test/tasks.test.js
  18:29  error  "@google-cloud/datastore" is not found  node/no-missing-require

/tmpfs/src/git/autosynth/working_repo/samples/tasks.js
  23:29  error  "@google-cloud/datastore" is not found  node/no-missing-require

✖ 4 problems (4 errors, 0 warnings)

npm ERR! code ELIFECYCLE
npm ERR! errno 1
npm ERR! @google-cloud/datastore@3.1.1 fix: `gts fix && eslint '**/*.js' --fix`
npm ERR! Exit status 1
npm ERR! 
npm ERR! Failed at the @google-cloud/datastore@3.1.1 fix script.
npm ERR! This is probably not a problem with npm. There is likely additional logging output above.

npm ERR! A complete log of this run can be found in:
npm ERR!     /home/kbuilder/.npm/_logs/2019-03-05T12_12_21_214Z-debug.log
synthtool > Cleaned up 2 temporary directories.
synthtool > Wrote metadata to synth.metadata.

```

.kokoro/release/publish.cfg

@@ -18,6 +18,26 @@ before_action {
   }
 }
 
+# Fetch magictoken to use with Magic Github Proxy 
+before_action {
+  fetch_keystore {
+    keystore_resource {
+      keystore_config_id: 73713
+      keyname: "releasetool-magictoken"
+    }
+  }
+}
+
+# Fetch api key to use with Magic Github Proxy 
+before_action {
+  fetch_keystore {
+    keystore_resource {
+      keystore_config_id: 73713
+      keyname: "magic-github-proxy-api-key"
+    }
+  }
+}
+
 # Download trampoline resources.
 gfile_resources: "/bigstore/cloud-devrel-kokoro-resources/trampoline"
 

synth.metadata

@@ -1,5 +1,5 @@
 {
-  "updateTime": "2019-02-26T12:27:48.396151Z",
+  "updateTime": "2019-03-05T12:12:21.223816Z",
   "sources": [
     {
       "generator": {
@@ -12,15 +12,15 @@
       "git": {
         "name": "googleapis",
         "remote": "https://github.com/googleapis/googleapis.git",
-        "sha": "29f098cb03a9983cc9cb15993de5da64419046f2",
-        "internalRef": "235621085"
+        "sha": "b4a22569c88f1f0444e889d8139ddacb799f287c",
+        "internalRef": "236712632"
       }
     },
     {
       "template": {
         "name": "node_library",
         "origin": "synthtool.gcp",
-        "version": "2019.1.16"
+        "version": "2019.2.26"
       }
     }
   ],