chores(build): run codecov on continuous builds (#476)

Author: jkwlui

.kokoro/continuous/node8/test.cfg

@@ -0,0 +1,9 @@
+# Bring in codecov.io master token into the build as $KOKORO_KEYSTORE_DIR/73713_dpebot_codecov_token
+before_action {
+  fetch_keystore {
+    keystore_resource {
+      keystore_config_id: 73713
+      keyname: "dpebot_codecov_token"
+    }
+  }
+}