Javadoc
This method is here so that when invoked, HdfsConfiguration is class-loaded
if it hasn't already been previously loaded. Upon loading the class, the
static initializer block above will be executed to add the deprecated keys
and to add the default resources. It is safe for this method to be called
multiple times as the static initializer block will only get invoked once.
This replaces the previously, dangerous practice of other classes calling
Configuration.addDefaultResource("hdfs-default.xml") directly without
loading this class first, thereby skipping the key deprecation.