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
(cherry picked from commit 97cd1731)