swift storyboard

General > Deployment Info > Main Interface でLogin.storyboardを選択してください。 

Storyboardコードでの指定

import UIKit

@UIApplicationMain
class AppDelegate: UIResponder, UIApplicationDelegate {

    var window: UIWindow?

    func application(application: UIApplication, didFinishLaunchingWithOptions launchOptions: [NSObject: AnyObject]?) -> Bool {

        let storyboard = UIStoryboard.init(name: "Login", bundle: nil)
        let loginViewController = storyboard.instantiateInitialViewController()

        self.window!.rootViewController = loginViewController

        self.window!.backgroundColor = UIColor.whiteColor()
        self.window!.makeKeyAndVisible()

        return true
    }
}