Add default variable in role/pve-subscribe

A new inventory variable `pve_subcription_key` now controls wether the enterprise
or no-subscription package repository is used.

roles/pve-subscribe/defaults/main.yml

@@ -0,0 +1 @@
+pve_subscription_key: Null

roles/pve-subscribe/tasks/main.yml

@@ -6,8 +6,8 @@
 
 - name: Configuring pve for unsubscribed operation
   import_tasks: no-subscription.yml
-  when: pve_enterprise | default(False) != True
+  when: pve_subscription_key | length == 0
 
 - name: Configuring pve for licensed operation
   import_tasks: enterprise.yml
-  when: pve_enterprise | default(False) == True
+  when: pve_subscription_key | length != 0