Update cdn role for buster

This adds the packages libjs-bootstrap4 and libjs-chart.js, which are
only available from buster onwards.

cdn/tasks/main.yml

 ---
 
+- include_vars:
+    file: "{{debian_version|default('fallback')}}.yml"
+
 - name: install commonly used web libraries
   apt:
-    name:
-      - libjs-jquery
-      - libjs-bootstrap
-      - libjs-jquery-datatables
-      - libjs-jquery-ui
+    name: "{{cdn_packages}}"
     state: present
-  tags:
-    - cdn

cdn/vars/buster.yml

+---
+
+cdn_packages:
+  - libjs-bootstrap
+  - libjs-bootstrap4
+  - libjs-chart.js
+  - libjs-jquery
+  - libjs-jquery-datatables
+  - libjs-jquery-ui

cdn/vars/fallback.yml

+---
+
+cdn_packages:
+  - libjs-jquery
+  - libjs-bootstrap
+  - libjs-jquery-datatables
+  - libjs-jquery-ui