[generators] advertise kadeploy as tftp server instead of kavlan gateway

generators/puppet/templates/kavlan-dhcp.conf.erb

@@ -25,7 +25,7 @@ subnet <%= kavlan_ip.network.to_addr %> netmask <%= kavlan_ip.netmask.to_addr %>
     option subnet-mask <%= kavlan_ip.netmask.to_addr %>;
     option broadcast-address <%= kavlan_ip.broadcast.to_addr %>;
     filename  "pxelinux.0";
-    next-server <%= kavlan_id.to_i.between?(1, 3)? kavlan['gateway'] : "kadeploy.#{site_uid}.grid5000.fr" %>;
+    next-server <%= "kadeploy.#{site_uid}.grid5000.fr" %>;
 
 <%
 refapi['sites'].sort.to_h.each_key do |site|