CDH-18512: Modification to allow spaces around the = sign in SET in impala-shell

Change-Id: I3c149e9a27962ed1130b1ddbb02952f4254bd4c9
Reviewed-on: http://gerrit.ent.cloudera.com:8080/2609
Reviewed-by: Taras Bobrovytsky <tbobrovytsky@cloudera.com>
Tested-by: jenkins