Separating ONC<->Shill translation from the ONC signature.

chromeos/test/data/network/translation_of_shill_l2tpipsec.onc

+{ "GUID": "guid",
+  "Type": "VPN",
+  "Name": "MyL2TPVPN",
+  "VPN": {
+    "Type": "L2TP-IPsec",
+    "Host": "some.host.org",
+    "IPsec": {
+      // These two fields are part of the ONC (and are required). However, they
+      // don't exist explicitly in the Shill dictionary. As there is no use-case
+      // yet, that requires to reconstruct these fields from a Shill dictionary,
+      // we don't require their translation.
+      // "AuthenticationType": "PSK",
+      // "IKEVersion": 1,
+      "PSK": "some_preshared_key",
+      "SaveCredentials": true
+    },
+    "L2TP": {
+      "Username": "some username",
+      "Password": "some password"
+    }
+  }
+}