Fix broken list concatenation in horizon role

Starting with ansible-core 2.13, list concatenation format is changed
and does not support concatenation operations outside of the jinja template.

The format change:

  "[1] + {{ [2] }}" -> "{{ [1] + [2] }}"

This affects the horizon role that iterates over existing policy files to
override and concatenate them into a single variable.

Co-Authored-By: Dr. Jens Harbott <harbott@osism.tech>

Closes-Bug: #2045660
Change-Id: I91a2101ff26cb8568f4615b4cdca52dcf09e6978

ansible/roles/horizon/tasks/policy_item.yml

@@ -20,6 +20,6 @@
 
 - name: Update custom policy file name
   set_fact:
-    custom_policy: "{{ custom_policy }} + [ '{{ overwritten_files.results.0.stat.path }}' ]"
+    custom_policy: "{{ custom_policy + [overwritten_files.results.0.stat.path] }}"
   when:
     - overwritten_files.results

releasenotes/notes/bug-2045660-inability-to-override-horizon-policy-files-c405906a9faf8f3b.yaml

+---
+fixes:
+  - |
+    Starting with ansible-core 2.13, list concatenation format is changed
+    which resulted in inability to override horizon policy files.
+    See `LP#2045660 <https://launchpad.net/bugs/2045660>`__
+    for more details.