[Analytics] Hadoop cluster - Added automatic failover to the HDFS Namenode

Hi! TL;DR: The Analytics team added automatic failover for the HDFS Namenode. A new daemon is running on the analytics1001/1002 hosts called hadoop-hdfs-zkfc (port 8019) responsible to talk with Zookeeper and execute periodical health checks. Monitoring and Ferm rules has been added. More info: 1) https://phabricator.wikimedia.org/T129838 2) https://hadoop.apache.org/docs/r2.6.0/hadoop-project-dist/hadoop-hdfs/HDFSH… 3) Monitoring and Ferm rules added: https://gerrit.wikimedia.org/r/#/c/279408/ Let me know if you have any questions! Luca