lib/plugins/aws/provider/awsProvider.js/AwsProvider/getStageSourceValue
class AwsProvider:  getStageSourceValue() {
    const values = this.getValues(this, [
      ['options', 'stage'],
      ['serverless', 'config', 'stage'],
      ['serverless', 'service', 'provider', 'stage'],
    ]);
    return this.firstValue(values);
  }