lib/classes/Variables.js/Variables/populateVariable
class Variables:  populateVariable(propertyParam, matchedString, valueToPopulate) {
    let property = propertyParam;
    if (typeof valueToPopulate === 'string') {
      property = replaceall(matchedString, valueToPopulate, property);
    } else {
      if (property !== matchedString) {
        if (typeof valueToPopulate === 'number') {
          property = replaceall(matchedString, String(valueToPopulate), property);
        } else {
          const errorMessage = [
            'Trying to populate non string value into',
            ` a string for variable ${matchedString}.`,
            ' Please make sure the value of the property is a string.',
          ].join('');
          throw new this.serverless.classes
            .Error(errorMessage);
        }
        return BbPromise.resolve(property);
      }
      property = valueToPopulate;
    }
    return BbPromise.resolve(property);
  }