Fix secret key handling in s3n:// (HADOOP-10511)

https://issues.apache.org/jira/browse/HADOOP-10511

I forgot to mention in the commit why I'm adding "example.com" as the default bucket. "s3n:///" is not a valid path, and the URI constructor throws a URISyntaxException for it. I think it is not a great idea anyway to use an invalid path in the tests. I understand the intention of not using something that may be a real bucket, because it could result in accidental damage to production data. But "example.com" is perfect for this, since it cannot exist.

Let me know if you have other questions about the change. Thanks!