[airos] set default value for sys keyword when not present

Author: edoput

netjsonconfig/backends/airos/converters.py

@@ -196,6 +196,7 @@ def ebtables(self):
             })
             vlans.append(t)
         if vlans:
+            base.setdefault('sys', {})
             base['sys']['vlan.status'] = 'enabled'
             base['sys']['vlan'] = vlans
         return [ebtables_status, base]