Add Virtual-Template1 configuration for LAC L2TP (#1693)

Add missed `Virtual-Template1` for LAC L2TP Cisco config example

(cherry picked from commit ebb33be)

Author: sever-sever

docs/configexamples/lac-lns.rst

@@ -67,6 +67,14 @@ LAC
      media-type rj45
      pppoe enable group MAIN-BBA
     !
+    interface Virtual-Template1
+     description pppoe MAIN-BBA
+     no ip address
+     no peer default ip address
+     ppp mtu adaptive
+     ppp authentication chap
+    !
+
 
 LNS
 ---