Add default value for cluster_public_protocol

Change-Id: I1240c175b59da676013383a44343cd21b0d2d356
diff --git a/heat/server/cluster.yml b/heat/server/cluster.yml
index 7edb7b0..ee5df33 100644
--- a/heat/server/cluster.yml
+++ b/heat/server/cluster.yml
@@ -4,6 +4,8 @@
 - service.keepalived.cluster.single
 - system.haproxy.proxy.listen.openstack.heat
 parameters:
+  _param:
+    cluster_public_protocol: https
   heat:
     server:
       stack_domain_admin: