[haproxy-config] Fix missing servers in split cfg

When using the split config style, all backends would be empty, which
meant that HAProxy was unable to serve any traffic. This turned out to
be due to a bad default in the split config template.

Closes-Bug: #1872545
Change-Id: I952e526e735e1d31445963f04d41d66bbdbfdee4

ansible/roles/haproxy-config/templates/haproxy_single_service_split.cfg.j2

@@ -108,7 +108,7 @@ backend {{ service_name }}_back
         {# services can be listening on a different port than haproxy #}
         {% set listen_port = haproxy_service.listen_port|default(haproxy_service.port) %}
         {# Custom member list can use jinja to generate a semicolon separated list #}
-        {% set custom_member_list = haproxy_service.custom_member_list|default() %}
+        {% set custom_member_list = haproxy_service.custom_member_list|default(none) %}
         {# Mode defaults to http #}
         {% set mode = haproxy_service.mode|default('http') %}
         {# Use the parent host group but allow it to be overridden #}

releasenotes/notes/bug-1872545-52f00bd340a800c2.yaml

+---
+fixes:
+  - |
+    Fixes issue where HAProxy would have no backend servers in its config files
+    when using the "split" config template style.