Fixes bind9 restarting when no designate_forwarders_addresses

When deploying designate if no designate_forwarders_addresses
is provided, bind9 container keeps restarting due an invalid(empty)
forwarder addresses is set in named.conf

Change-Id: I7d309eb077243435dd2038629074251abec7d3e1
Closes-Bug: #1787092

ansible/roles/designate/templates/named.conf.j2

+#jinja2: trim_blocks: False
 include "/etc/rndc.key";
 options {
         listen-on port {{ designate_bind_port }} { {{ hostvars[inventory_hostname]['ansible_' + hostvars[inventory_hostname]['api_interface']]['ipv4']['address'] }}; };
@@ -7,7 +8,9 @@ options {
         auth-nxdomain no;
         request-ixfr no;
         recursion {{ designate_recursion }};
+        {% if designate_forwarders_addresses %}
         forwarders { {{ designate_forwarders_addresses }}; };
+        {% endif %}
         minimal-responses yes;
         allow-notify { {% for host in groups['designate-worker'] %}{{ hostvars[host]['ansible_' + hostvars[host]['api_interface']]['ipv4']['address'] }};{% endfor %} };
 };