lib/plugins/aws/provider/awsProvider.js/AwsProvider/getCredentials
class AwsProvider:  getCredentials(stage, region) {
    const ret = { region };
    const credentials = {};
    const stageUpper = stage ? stage.toUpperCase() : null;

    // add specified credentials, overriding with more specific declarations
    impl.addCredentials(credentials, this.serverless.service.provider.credentials); // config creds
    impl.addProfileCredentials(credentials, this.serverless.service.provider.profile);
    impl.addEnvironmentCredentials(credentials, 'AWS'); // creds for all stages
    impl.addEnvironmentProfile(credentials, 'AWS');
    impl.addEnvironmentCredentials(credentials, `AWS_${stageUpper}`); // stage specific creds
    impl.addEnvironmentProfile(credentials, `AWS_${stageUpper}`);

    if (Object.keys(credentials).length) {
      ret.credentials = credentials;
    }
    return ret;
  }