auth: Refactor how authentication methods are passed to server/auth library.

appengine/gaeauth/server/prpc.go

 // Copyright 2016 The LUCI Authors. All rights reserved.
 // Use of this source code is governed under the Apache License, Version 2.0
 // that can be found in the LICENSE file.
 
 package server
 
 import (
         "github.com/luci/luci-go/grpc/prpc"
         "github.com/luci/luci-go/server/auth"
 )
 
 func init() {
-        prpc.RegisterDefaultAuth(auth.Authenticator{
-                &OAuth2Method{Scopes: []string{EmailScope}},
+        prpc.RegisterDefaultAuth(&auth.Authenticator{
+                Methods: []auth.Method{
+                        &OAuth2Method{Scopes: []string{EmailScope}},
+                },
         })
 }