This is initially intended for storing GCI image versions.

Example output:

    $ TEST_STARTED_JSON=1 BUILD_METADATA_GCE_IMAGE=m53 \
    BUILD_METADATA_VARIANT=reliableforreal BUILD_NUMBER=123 \
    WORKSPACE="blaga" JOB_NAME="test-e2e" hack/jenkins/upload-to-gcs.sh
    {
      "version": "v1.5.0-alpha.0.806+fa943089-dirty",
      "timestamp": 1473894301,
      "metadata": {
        "gce_image": "m53",
        "variant": "reliableforreal"
      },
      "jenkins-node": ""
    }