'Fetches a field from extras, and returns it. This is some Airflow
magic. The google_cloud_platform hook type adds custom UI elements
to the hook page, which allow admins to specify service_account,
key_path, etc. They get formatted as shown below.'

def _get_field(self, f, default=None):

    long_f = 'extra__google_cloud_platform__{}'.format(f)
    if (long_f in self.extras):
        return self.extras[long_f]
    else:
        return default
