…ost qualified hdfs principal name.

Perform a DNS reverse name lookup when getCanonicalHost() returns the IP address as a string.

Since the core of the problem relates to DNS interactions with private fields inside of Java language classes, this test can only verify that externally a request for a canonical host name works.

Instead of introducing a new utility class (i.e. InetAddressUtils) switch to using the existing pluggable framework for DomainNameResolver.

Update default implementation DNSDomainNameResolver to protected against returning the IP address as a string from a cached value.

 SpotBugs is concerned that getCanonicalHostName() may return null, although the JavaDocs indicate that it will return the IP address as a String if unable to determine the FQDN.  The only circumstance I can imagine where it would return null is if the IP address is null, which would mean a reverse lookup wouldn't work either.