lib/classes/Variables.js/Variables/getValueFromS3
class Variables:  getValueFromS3(variableString) {
    const groups = variableString.match(this.s3RefSynax);
    const bucket = groups[1];
    const key = groups[2];
    return this.serverless.getProvider('aws')
    .request('S3',
      'getObject',
      {
        Bucket: bucket,
        Key: key,
      },
      this.options.stage,
      this.options.region)
    .then(
      response => response.Body.toString(),
      err => {
        const errorMessage = `Error getting value for ${variableString}. ${err.message}`;
        throw new this.serverless.classes.Error(errorMessage);
      }
    );
  }