Use SPF Viper for configuration

Closes #68 (closed)

cmd/gnmi/gnmi.go

 package main
 
 import (
+	"code.fbi.h-da.de/cocsn/gosdn/forks/goarista/gnmi"
 	"code.fbi.h-da.de/cocsn/gosdn/nucleus"
 	"context"
 	"fmt"
@@ -24,7 +25,13 @@ func main() {
 		log.Fatal(err)
 	}
 
-	ctx := context.WithValue(context.Background(), "config", device.Config)
+	cfg := &gnmi.Config{
+		Addr:     device.Config.Address,
+		Password: device.Config.Password,
+		Username: device.Config.Username,
+	}
+	ctx := gnmi.NewContext(context.Background(), cfg)
+	ctx = context.WithValue(ctx, "config", cfg)
 
 	g := nucleus.Gnmi{}
 	resp, err := g.Capabilities(ctx)