IMPALA-921: Change EXPLAIN_LEVEL value from 0 to 1 in impala-shell for SET command

Change-Id: I2bfcefb5c8143d4cb4d74157c5309cd9445bac02
Reviewed-on: http://gerrit.ent.cloudera.com:8080/3383
Reviewed-by: Abdullah Yousufi <abdullah.yousufi@cloudera.com>
Tested-by: jenkins