Only restore from S3 if backups exist.

* If the S3 bucket doesn't contain a backup of the Minecraft
  data files, then skip the restore step. In that case, server
  will start up with the Minecraft world created during the
  AMI creation.
* Call different restore script in msm-s3-sync.
* Remove backup_to_s3() task in build-ami.py. No longer save newly
  created Minecraft world to S3. That way, it won't overwrite a
  Minecraft world already saved to S3.

Fixes #7.