Should provide flag for using system TLS certs

[torkelrogstad]

I have a LND node running behind TLS that's accepted by my system without having to specify a custom TLS cert path. This is currently very hard to use with Faraday out of the box. I got it to work by commenting out line 77 in faraday.go, and adding SystemCert: true to the lndclient.LndServicesConfig struct.

faraday/faraday.go

Line 77 in bb86712

TLSPath: config.Lnd.TLSCertPath,

It would be a good idea to expose this functionality through CLI flags.