Handle cases where environment variables are not defined in impala-config.sh

Change-Id: Iee2800cb02299a9ed26da6fd079e3a72fe2a2482
Reviewed-on: http://gerrit.ent.cloudera.com:8080/2537
Reviewed-by: Lenni Kuff <lskuff@cloudera.com>
Tested-by: Lenni Kuff <lskuff@cloudera.com>