code: public static void checkOptional(OptionSet options, String opt1, String opt2) {
          List<String> opts = Lists.newArrayList();
          opts.add(opt1);
          opts.add(opt2);
          checkOptional(options, opts);
      }
docs: Checks if there's at most one option that exists among all opts.
  
  @param parser OptionParser to checked
  @param opt1 Optional option to check
  @param opt2 Optional option to check
  @throws VoldemortException