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