Add DEFAULT_QD_MAX_CONNECTION to 250

This is to pass isolation2 max concurrency tests.

Author: Xin Zhang <xzhang@pivotal.io>
Author: Jimmy Yih <jyih@pivotal.io>

gpAux/gpdemo/Makefile

@@ -37,6 +37,7 @@ export NUM_PRIMARY_MIRROR_PAIRS
 export WITH_MIRRORS
 export WITH_STANDBY
 export BLDWRAP_POSTGRES_CONF_ADDONS
+export DEFAULT_QD_MAX_CONNECT=250
 
 all:
 	$(MAKE) clean