Ensure we set $haproxy_firewall_rules variable

If we do not set a default value for that one, we might hit the
following puppet warning:

Warning: Unknown variable: 'haproxy_firewall_rules'. (file:
/etc/puppet/modules/tripleo/manifests/haproxy/endpoint.pp, line: 303,
column: 54)

Change-Id: I6eb75c88e06281c61e4317fe8722e4bdfa572fe4
(cherry picked from commit 34bf20d668)

manifests/haproxy/endpoint.pp

@@ -284,6 +284,8 @@ define tripleo::haproxy::endpoint (
           'dport' => $haproxy_port_real,
         },
       }
+    } else {
+      $haproxy_firewall_rules = {}
     }
     if $public_ssl_port {
       $haproxy_ssl_firewall_rules = {