GbqConnector should be able to fetch default credentials on Google Compute Engine
https://github.com/pydata/pandas/blob/master/pandas/io/gbq.py

```
class GbqConnector(object):
    ...
    def get_user_account_credentials(self):
        ...
    ...
```

Google Compute Engine and Google Dataproc etc. already have default application credentials on them. Therefore, there is no need of running `OAuth2WebServerFlow` while the method `get_user_account_credentials` is being called - instead `return GoogleCredentials.get_application_default()` is all you need. This change will allow you to run `GbqConnector` on commandline-only systems on Google Compute Engine without a need of a service account json key.
