New returns an implementation of the config service which talks to the actual luci-config service using given transport.
configServiceURL is usually "https://<host>/_ah/api/config/v1/".
ClientFactory returns http.Clients to use for requests (given incoming contexts). It's required mostly to support GAE environment, where round trippers are bound to contexts and carry RPC deadlines.
If 'clients' is nil, http.DefaultClient will be used for all requests.