Add an ONC property for disabling LCP echo

chromeos/test/data/network/translation_of_shill_l2tpipsec.onc

 { "GUID": "guid",
   "Type": "VPN",
   "Name": "MyL2TPVPN",
   "VPN": {
     "Type": "L2TP-IPsec",
     "Host": "some.host.org",
     "IPsec": {
       "AuthenticationType": "PSK",
       // This field is part of ONC (and is required). However, it does not
       // exist explicitly in the Shill dictionary. As there is no use-case yet
       // that requires reconstructing this field from a Shill dictionary, we
       // don't translate it.
       // "IKEVersion": 1,
       "PSK": "some_preshared_key",
       "SaveCredentials": true,
       "XAUTH": {
           "Username": "some xauth username",
           "Password": "some xauth password"
       }
     },
     "L2TP": {
       "Username": "some username",
       "Password": "some password",
+      "LcpEchoDisabled": true,
     },
   }
 }