SPARK-1767: Prefer HDFS-cached replicas when scheduling data-local tasks

This change reorders the replicas returned by
HadoopRDD#getPreferredLocations so that replicas cached by HDFS are at
the start of the list.  This requires Hadoop 2.5 or higher; previous
versions of Hadoop do not expose the information needed to determine
whether a replica is cached.
The contract of this method is kinda sketchy -- taking in a "str" which is either a host name or a tag. Would you mind adding a bit of Javadoc to explain that this is what is happening?