Use primary_endpoint_addess if dns is not used for host output (#38)

Fixes #37

output.tf

@@ -14,6 +14,6 @@ output "port" {
 }
 
 output "host" {
-  value       = "${module.dns.hostname}"
+  value       = "${coalesce(module.dns.hostname, join("", aws_elasticache_replication_group.default.*.primary_endpoint_address))}"
   description = "Redis host"
 }