HDFS-5314.  Do not expose CachePool type in AddCachePoolOp (Colin Patrick McCabe)

git-svn-id: https://svn.apache.org/repos/asf/hadoop/common/branches/HDFS-4949@1530073 13f79535-47bb-0310-9956-ffa450edef68
HDFS-5224: introduce a builder for PathBasedCacheDirective
Merge branch 'HDFS-4949' into HDFS-5224
HDFS-5224: minor clean-ups