main/lntest: factor out node config options, add options for bitcoind

This commit factors out the btcd and ltcd options into their own sections
similar to neutrino, and adds a bitcoind section as well. Now, you specify
node options similarly to:

--ltcd.rpchost=...
or
--btcd.rpcuser=...
or
--bitcoind.zmqpath=...

For Bitcoin, you specify an alternate back-end to btcd as follows:

--bitcoin.node=bitcoind
or
--bitcoin.node=neutrino

You can also specify the default option:
--bitcoin.node=btcd

For Litecoin, only `btcd` mode is valid, and corresponds to the `ltcd`
section. For example:

--litecoin.node=btcd
--ltcd.rpchost=...

The new code also attempts to read the correct options and auth info
from bitcoin.conf just as it does from btcd.conf/ltcd.conf.

lnd.go

@@ -31,8 +31,8 @@ import (
 	"google.golang.org/grpc"
 	"google.golang.org/grpc/credentials"
 
-	flags "github.com/btcsuite/go-flags"
 	proxy "github.com/grpc-ecosystem/grpc-gateway/runtime"
+	flags "github.com/jessevdk/go-flags"
 	"github.com/lightningnetwork/lnd/autopilot"
 	"github.com/lightningnetwork/lnd/channeldb"
 	"github.com/lightningnetwork/lnd/lnrpc"