Add missing LB_SUBDOMAIN for dual-master config

The dual-master receipe was missing the LB_SUBDOMAIN in
the setup.env.template, making the service-lb stack to
fail during creation.

Change-Id: I87026cd6f709d24b3b461f5319cca0dedfd075a9
diff --git a/dual-master/setup.env.template b/dual-master/setup.env.template
index 472cf07..6858912 100644
--- a/dual-master/setup.env.template
+++ b/dual-master/setup.env.template
@@ -10,5 +10,6 @@
 HOSTED_ZONE_NAME:=yourcompany.com
 MASTER1_SUBDOMAIN:=$(AWS_PREFIX)-master-1.gerrit-demo
 MASTER2_SUBDOMAIN:=$(AWS_PREFIX)-master-2.gerrit-demo
+LB_SUBDOMAIN=$(AWS_PREFIX)-lb.gerrit-demo
 DOCKER_REGISTRY_URI:=<yourAccountId>.dkr.ecr.us-east-1.amazonaws.com
 SSL_CERTIFICATE_ARN=arn:aws:acm:us-east-1:<yourAccountId>:certificate/33e2c235-a4d1-42b7-b866-18d8d744975c