playbook: Fix variable ordering on slapos.yml

  vars_files overwrites var_prompt inputs, so keep it minimal on slapos installation.

playbook/slapos.yml

@@ -2,8 +2,8 @@
   hosts: 127.0.0.1
   connection: local
 
-  vars_files:
-    - settings/vifib.yml
+  vars:
+    - interface_name: lo
 
   vars_prompt:
     - name: "slapos_master_url"