Fix a bug in creating the demo cluster

This bug was introduced in fd2e045f.

Thanks to Shoaib Lari for pointing this out.

gpAux/gpdemo/demo_cluster.sh

@@ -4,7 +4,7 @@
 # Data Directories
 # ======================================================================
 
-DATADIRS?=`pwd`/datadirs
+DATADIRS=${DATADIRS:-`pwd`/datadirs}
 QDDIR=$DATADIRS/qddir
 
 SEG_PREFIX=demoDataDir