Remove the use of vars and hostvars wherever possible.

`vars` operates inconsistently in different combinations of
Ansible (full/base/core) and Python versions.

Variables are now instead explicitly loaded under a subkey, which also removes
the need for filtering and makes access more explicit and semantic.

roles/install_demo/tasks/load_vars.yml

@@ -2,7 +2,8 @@
 - name: include all demo vars from install_demo roll
   include_vars:
     dir: "vars"
+    name: job_template_definitions
 
-- name: will load vars from adjacent vars
+- name: will load vars
   debug:
-    msg: "variables loaded from install_demo role"
+    msg: "variables loaded from install_demo role into job_template_definitions"