Add an ONC property for disabling LCP echo

chromeos/test/data/network/l2tpipsec_clientcert_with_cert_pems.onc

 { "GUID": "guid",
   "Type": "VPN",
   "Name": "MyL2TPVPN",
   "VPN": {
       "Type": "L2TP-IPsec",
       "Host": "some.host.org",
       "IPsec": {
           "AuthenticationType": "Cert",
           "ClientCertType": "Pattern",
           "ClientCertPattern": {
@@ skipping 11 matching lines @@
           },
           "IKEVersion": 1,
           "SaveCredentials": true,
           "XAUTH": {
               "Username": "some xauth username",
               "Password": "some xauth password"
           }
       },
       "L2TP": {
           "Username": "some username",
-          "Password": "some password"
+          "Password": "some password",
+          "LcpEchoDisabled": true
       }
   }
 }