IMPALA-1553: Fix sequence of UnionNode.init() calls for single node execution.

Change-Id: I6fe909f6cad30d9f594f52034e13a83156750a17
Reviewed-on: http://gerrit.sjc.cloudera.com:8080/5438
Reviewed-by: Alex Behm <alex.behm@cloudera.com>
Tested-by: jenkins