config: add regtest support to lnd

Support for regtest allows us to create integration tests to verify
interoperability with other implementations. This seems to work, and
allows to connect to `btcd` and indirectly to `bitcoind` so we can run
lnd on the same network as eclair and c-lightning.

For details about my integration framework check this repo:
cdecker/lightning-integration

params.go

@@ -45,6 +45,12 @@ var liteTestNetParams = litecoinNetParams{
 	rpcPort: "19334",
 }
 
+// regTestNetParams contains parameters specific to a local regtest network.
+var regTestNetParams = bitcoinNetParams{
+	Params:  &bitcoinCfg.RegressionNetParams,
+	rpcPort: "18334",
+}
+
 // applyLitecoinParams applies the relevant chain configuration parameters that
 // differ for litecoin to the chain parameters typed for btcsuite derivation.
 // This function is used in place of using something like interface{} to