Fix cloudformation load balancer ouput (Sage-Bionetworks-IT#4)

We depend on the cloudformation's load balancer exported DNS endpoint
to configure a DNS redirect in organizations-infra repo[1].  The export
name needs to be unique in the account.

[1] https://github.com/Sage-Bionetworks-IT/organizations-infra/blob/master/org-formation/800-redirects/_tasks.yaml

src/load_balancer_stack.py

@@ -21,4 +21,9 @@ def __init__(
         self.alb = elbv2.ApplicationLoadBalancer(
             self, "AppLoadBalancer", vpc=vpc, internet_facing=True
         )
-        cdk.CfnOutput(self, "dns", value=self.alb.load_balancer_dns_name)
+        cdk.CfnOutput(
+            self,
+            "LoadBalancerDns",
+            value=self.alb.load_balancer_dns_name,
+            export_name=f"{construct_id}-dns",
+        )