code: function redisCheckCertificateKeypair(options, callback) {
      _debug('le-store-redis.redisCheckCertificateKeypair:', options.domains);
  
      if(options.domains && options.domains[0]) {
        return _getByIndex('idx-d2k', options.domains[0], callback);
      }
  
      callback(new Error('le-store-redis.redisCheckCertificateKeypair requires ' +
        'options.domains'));
    }
docs: Retrieves a keypair associated with a certificate from the database.
  
  @param {Object[]} options - options passed to storage call
  @param {string} options[].domains - an array of domains that may be
  associated with the certificate keypair. Only the first domain is used.
  @param {Function} callback(err, keypair) - called after storage attempt,
  keypair will be null if it was not found.