If we only set the --datadir flag then lnd will create empty directories
(for letsencrypt and watchtower client) in the default user's home .lnd
directory and will also pick up a config file from there if one exists.
To instruct lnd to only act within the temp itest directory, we need to
use the --lnddir flag instead.
Using the --lnddir flag also overwrites the TLS cert/key path and the
log dir, so we don't need to explicitly set those anymore.
But we need to make sure the node's variables point to the correct
default files/directories.
This commit adds a new struct RPCClients to better handle rpc clients.
A private field, rpc, is added to HarnessNode to prevent direct access
to its clients. Inside RPCClients, all clients are exported in case a
test case need access to a specific client.
