Changes for HDFS-3042 branch.

This change list will be merged into the trunk CHANGES.txt when the HDFS-3042
branch is merged.
------------------------------

HDFS-2185. HDFS portion of ZK-based FailoverController (todd)

HDFS-3200. Scope all ZKFC configurations by nameservice (todd)

HDFS-3223. add zkfc to hadoop-daemon.sh script (todd)

HDFS-3261. TestHASafeMode fails on HDFS-3042 branch (todd)

HDFS-3159. Document NN auto-failover setup and configuration (todd)