I'd prefer to use `urllib2` in this case, for three reasons:

 * First and foremost, I eventually want to drop the dependency on `httplib2` in this library. No point in adding more references.

 * As a symptom of why I want to drop it, last week the GCS folks hit some strange issues with `httplib2` in this particular codepath (in apitools, but it's doing the same thing). We didn't dig much, but switching to `urllib2` fixed it.

 * Given that we're testing for GCE anyway, I don't see an important use case for the user passing in a custom http instance.