Fixed prepare.sh script to find the external net name

PROD: PROD-36810
Change-Id: Ib09cdd2d13f52919477dd36cd6ae272185538ed5
diff --git a/scripts/prepare.sh b/scripts/prepare.sh
index c0b58ba..9e3e78f 100644
--- a/scripts/prepare.sh
+++ b/scripts/prepare.sh
@@ -293,8 +293,8 @@
     echo router add subnet ${router_id} ${subnet2_id} >>${cmds}
     process_cmds
 
-    # TODO: Search for external net
-    external=prod-public
+    # get external network name
+    external=$(openstack network list --external -c Name -f value | head -n1)
     echo router set ${router} --external-gateway ${external} >>${cmds}
     process_cmds
 }