lib/classes/Variables.js/Variables/prepopulateService
class Variables:  prepopulateService() {
    const provider = this.serverless.getProvider('aws');
    if (provider) {
      const requiredConfigs = [
        _.assign({ name: 'region' }, provider.getRegionSourceValue()),
        _.assign({ name: 'stage' }, provider.getStageSourceValue()),
      ];
      return this.disableDepedentServices(() => {
        const prepopulations = requiredConfigs.map(config =>
          this.populateValue(config.value, true) // populate
            .then(populated => _.assign(config, { populated })));
        return this.assignProperties(provider, prepopulations);
      });
    }
    return BbPromise.resolve();
  }