platforms/openstack/neutron/main.tf: Use DNS names for etcd endpoints

TLS certs for etcd are generated based on this variable. Previously, this
was using the bare IP addresses rather than DNS names, which is less
robust, and was causing TLS failures for external etcd.

platforms/openstack/neutron/main.tf

@@ -89,7 +89,7 @@ module "bootkube" {
 
   etcd_backup_size          = "${var.tectonic_etcd_backup_size}"
   etcd_backup_storage_class = "${var.tectonic_etcd_backup_storage_class}"
-  etcd_endpoints            = "${module.dns.etcd_a_nodes}"
+  etcd_endpoints            = "${data.template_file.etcd_hostname_list.*.rendered}"
   self_hosted_etcd          = "${var.tectonic_self_hosted_etcd}"
 
   master_count = "${var.tectonic_master_count}"