[Python 3] oauth exception (the JSON object must be str, not 'bytes') when using gcloud.storage

In https://github.com/google/oauth2client/blob/0aad0a09f50312e0fcc3a1234f0e7fb525010bfd/oauth2client/gce.py#L86, the code uses `d = json.loads(content)` to load `content` as JSON. The code is broken in Python 3 because `content` is binary, while `json.loads` expects string.
