Add an ONC property for disabling LCP echo

chromeos/test/data/network/shill_output_l2tpipsec.json

 { "GUID": "guid",
   "Type": "vpn",
   "Name": "MyL2TPVPN",
   "Provider": {
     "Host": "some.host.org",
+    "L2TPIPsec.LCPEchoDisabled": true,
     "L2TPIPsec.PSK": "some_preshared_key",
     "L2TPIPsec.User": "some username",
     "L2TPIPsec.XauthPassword": "some xauth password",
     "L2TPIPsec.XauthUser": "some xauth username",
     "L2TPIPsec.Password": "some password",
     "Type": "l2tpipsec",
   },
   "SaveCredentials": true
 }