Exclude disabled group from backup-server loop

We don't set bup_user for hosts in disabled in the backup role
if they are in the disabled group, so they are not available to
be used in backup-server. We need to also exclude them here.

Change-Id: Ib5fd249533d5d8de0179bf16d61cf88f4c11f301

playbooks/roles/backup-server/tasks/main.yaml

@@ -12,10 +12,10 @@
 - name: Build all bup users from backup hosts
   set_fact:
     bup_users: '{{ bup_users }} + [ {{ hostvars[item]["bup_user"] }} ]'
-  with_inventory_hostnames: backup
+  with_inventory_hostnames: 'backup:!disabled'
 
 - name: Create bup users
   include_tasks: user.yaml
   loop: '{{ bup_users }}'
   loop_control:
-    loop_var: bup_user
+    loop_var: bup_user