Allow default_backend to be specified

Currently default_backend for a service is always set to the
haproxy_service_name for a service, but this might not be what is
required for some configurations.

This patch allows haproxy_default_backend to be configured for
a service to customise the default_backend setting.

Depends-On: https://review.opendev.org/c/openstack/openstack-ansible-rabbitmq_server/+/876436
Change-Id: I9e2be37cb27a33350577a93f23b69e560493b320

defaults/main.yml

@@ -88,6 +88,7 @@ haproxy_service_configs: []
 #       haproxy_frontend_raw:
 #         - 'http-request use-service prometheus-exporter if { path /metrics }'
 #       haproxy_service_enabled: True
+#       haproxy_default_backend: other_backend
 
 galera_monitoring_user: monitoring
 haproxy_bind_on_non_local: False

releasenotes/notes/default_backend_override-9840dc75ff9d1a9c.yaml

@@ -0,0 +1,10 @@
+---
+features:
+  - |
+    A new key ``haproxy_default_backend`` can be defined for each service
+    configured in the haproxy loadbalancer. Configuring this variable writes
+    a value for the ``default_backend`` directive into the config for the
+    service frontend. It can be useful to provide a specific default backend
+    as a fall-through option when other backends are selected using ACLs,
+    and to also allow the name of the default backend to be different from
+    ``haproxy_service_name``.

templates/service.j2

@@ -83,8 +83,8 @@ frontend {{ item.service.haproxy_service_name }}-front-{{ loop.index }}
     http-request add-header X-Forwarded-Proto https
 {% endif %}
     mode {{ item.service.haproxy_balance_type }}
-{% if not item.service.haproxy_frontend_only | default(false) %}
+{% if (not item.service.haproxy_frontend_only | default(false)) or ((item.service.haproxy_default_backend is defined) and (item.service.haproxy_default_backend | length > 0)) %}
-    default_backend {{ item.service.haproxy_service_name }}-back
+    default_backend {{ item.service.haproxy_default_backend | default(item.service.haproxy_service_name) }}-back
 {% endif %}
 {% for entry in (item.service.haproxy_frontend_raw|default([])) + haproxy_frontend_extra_raw %}
     {{ entry }}