Debugger sample only activates when GCLOUD_PROJECT is defined

https://github.com/GoogleCloudPlatform/nodejs-docs-samples/blob/ba151d535beac1ca7dcabd7b2916acaf246b3432/debugger/app.js#L19

``` js
if (process.env.GCLOUD_PROJECT) {
  require('@google/cloud-debug');
}
```

What is the rationale for checking for `GCLOUD_PROJECT` being defined? On GCE and GKE, this env. var. is not going to be defined unless the customer explicitly defines it.
