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) {
        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);
      }
      property = valueToPopulate;
    }
    return property;
  }