feat: rename addon vars to addon_ prefix and simplify logic in jinja templates (#1216)

Signed-off-by: Devin Buhl <devin@buhl.casa>

bootstrap/templates/kubernetes/apps/tools/kustomization.yaml.j2

@@ -5,6 +5,6 @@ resources:
   - ./namespace.yaml
   - ./descheduler/ks.yaml
   - ./reloader/ks.yaml
-  #% if bootstrap_distribution == "k3s" and system_upgrade_controller|default({}) and system_upgrade_controller.enabled|default(false) %#
+  #% if bootstrap_distribution == "k3s" and addon_system_upgrade_controller.enabled %#
   - ./system-upgrade-controller/ks.yaml
   #% endif %#