lib/classes/Variables.js/Variables/populateVariable
class Variables:  populateVariable(propertyParam, matchedString, valueToPopulate) {
    let property = propertyParam;
    if (property === matchedString) {
      // total replacement
      property = valueToPopulate;
    } else if (_.isString(valueToPopulate)) {
      // partial replacement, string
      property = replaceall(matchedString, valueToPopulate, property);
    } else if (_.isNumber(valueToPopulate)) {
      // partial replacement, 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 property;
  }