Implement ability to define arbitrary options in local_settings.py

Unfortunatelly having `horizon_config_overrides` is not sufficient, as
not all options can be defined in HORIZON_CONFIG.
Some are expected to be preset inside local_settings themselves.

We introduce new variable that can be used to extend local_settings file
with arbitrary parameters.

Change-Id: Ic119105189c9158aa3b38c4deb07040110d9d660

defaults/main.yml

@@ -499,6 +499,7 @@ horizon_translations_pull: "{{ _horizon_translations_pull }}"
 
 # Set arbitrary horizon configuration options
 horizon_config_overrides: {}
+horizon_extra_local_settings: {}
 
 # Set overrides for horizon embedded policies
 # horizon_policy_overrides:

releasenotes/notes/horizon_extend_local_settings-9e062b515de94b24.yaml

@@ -0,0 +1,7 @@
+---
+features:
+  - |
+    Implemented variable ``horizon_extra_local_settings`` that allows to
+    insert arbitrary parameters defined as a regular mapping inside
+    local_settings.py of Horizon. Parameters will be inserted at the end of
+    the config file.

templates/horizon_local_settings.py.j2

@@ -842,6 +842,10 @@ DEFAULT_THEME = '{{ horizon_default_theme }}'
 # Set arbitrary horizon configuration options
 HORIZON_CONFIG.update({{ horizon_config_overrides }})
 
+{% for key, value in horizon_extra_local_settings.items() %}
+{{ key }} = {{ value }}
+{% endfor %}
+
 OPENSTACK_KEYSTONE_ADMIN_ROLES = [
 {% for item in horizon_keystone_admin_roles %}
   '{{ item }}',