Specify number of threads for designate bind9 backend

Bind9 is running without limit for UDP listeners.
This patch is changing this behaviour and sets max 32
of UDP listeners. This is needed because of bug below [1].

[1] https://bugs.launchpad.net/ubuntu/+source/bind9/+bug/1827923

Change-Id: Ie4c2ac4d5e990ebdc30c3a94d855703d814f1fee

ansible/roles/designate/defaults/main.yml

@@ -289,6 +289,7 @@ designate_producer_workers: "{{ openstack_service_workers }}"
 designate_central_workers: "{{ openstack_service_workers }}"
 designate_sink_workers: "{{ openstack_service_workers }}"
 designate_mdns_workers: "{{ openstack_service_workers }}"
+designate_backend_bind9_workers: "{{ [ansible_facts.processor_vcpus, 32] | min }}"
 
 ####################
 # Kolla

ansible/roles/designate/templates/designate-backend-bind9.json.j2

@@ -2,7 +2,7 @@
 {% set bind_file = 'bind/named.conf' if kolla_base_distro in ['ubuntu', 'debian'] else 'named.conf' %}
 
 {
-    "command": "/usr/sbin/{{ bind_cmd }} -g",
+    "command": "/usr/sbin/{{ bind_cmd }} -U {{ designate_backend_bind9_workers }} -g",
     "config_files": [
         {
             "source": "{{ container_config_directory }}/named.conf",

releasenotes/notes/new-variable-designate-backend-bind9-workers-cfd4676196410c6a.yaml

@@ -0,0 +1,5 @@
+---
+features:
+  - |
+    Adds support for configuring a number of UDP workers for Designate's
+    bind9 backend via the ``designate_backend_bind9_workers`` variable.