Add an ONC property for disabling LCP echo

chromeos/test/data/network/valid_l2tpipsec.onc

 { "GUID": "guid",
   "Type": "VPN",
   "Name": "MyL2TPVPN",
   "VPN": {
     "Type": "L2TP-IPsec",
     "Host": "some.host.org",
     "IPsec": {
       "AuthenticationType": "PSK",
       "IKEVersion": 1,
       "PSK": "some_preshared_key",
       "SaveCredentials": true,
       "XAUTH": {
           "Username": "some xauth username",
           "Password": "some xauth password"
       }
     },
     "L2TP": {
       "Username": "some username",
-      "Password": "some password"
+      "Password": "some password",
+      "LcpEchoDisabled": true
     }
   }
 }