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.
.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
synthtool > No replacements made in .circleci/config.yml for pattern command: npm run system-test, maybe replacement is not longer needed?
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 +1,12 @@
-{}
+{
+  "updateTime": "2019-03-05T12:25:56.275493Z",
+  "sources": [
+    {
+      "template": {
+        "name": "node_library",
+        "origin": "synthtool.gcp",
+        "version": "2019.2.26"
+      }
+    }
+  ]
+}