lightning: enable by default but without gossip

Enables lightning by creating a node private key and storing it in
the wallet. The gossiper is not launched at start up, only if there
are existing channels.

electrum/tests/test_wallet.py

@@ -163,6 +163,10 @@ class TestCreateRestoreWallet(WalletTestCase):
                               gap_limit=1,
                               config=self.config)
         wallet = d['wallet']  # type: Standard_Wallet
+
+        # lightning initialization
+        self.assertTrue(wallet.db.get('lightning_privkey2').startswith('xprv'))
+
         wallet.check_password(password)
         self.assertEqual(passphrase, wallet.keystore.get_passphrase(password))
         self.assertEqual(d['seed'], wallet.keystore.get_seed(password))