HDFS-7081. Add new DistributedFileSystem API for getting all the existing storage policies. Contributed by Jing Zhao.

Conflicts:
	hadoop-hdfs-project/hadoop-hdfs/CHANGES.txt
HDFS-7140. Add a tool to list all the existing block storage policies. Contributed by Jing Zhao.

Conflicts:
	hadoop-hdfs-project/hadoop-hdfs/CHANGES.txt
	hadoop-hdfs-project/hadoop-hdfs/src/main/bin/hdfs