HIVE-8395 : CBO: enable by default (Sergey Shelukhin, reviewed by Laljo John Pullokkaran, Ashutosh Chauhan, Jesus Camacho Rodriguez, Pengcheng Xiong)

git-svn-id: https://svn.apache.org/repos/asf/hive/trunk@1645808 13f79535-47bb-0310-9956-ffa450edef68