tokenserver: Boilerplate for loading and serving service_accounts.cfg.

tokenserver/appengine/impl/serviceaccounts/rpc_import_service_accounts_configs.go

 // Copyright 2017 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 serviceaccounts
 
 import (
         "golang.org/x/net/context"
         "google.golang.org/grpc"
         "google.golang.org/grpc/codes"
 
         "github.com/golang/protobuf/ptypes/empty"
+
+        "github.com/luci/luci-go/common/logging"
         "github.com/luci/luci-go/tokenserver/api/admin/v1"
 )
 
-// ImportServiceAccountsConfigsRPC implements Admin.ImportServiceAccountsConfigs
+// ImportServiceAccountsConfigsRPC implements admin.ImportServiceAccountsConfigs
 // method.
 type ImportServiceAccountsConfigsRPC struct {
+        RulesCache *RulesCache // usually GlobalRulesCache, but replaced in tests
 }
 
 // ImportServiceAccountsConfigs fetches configs from from luci-config right now.
 func (r *ImportServiceAccountsConfigsRPC) ImportServiceAccountsConfigs(c context.Context, _ *empty.Empty) (*admin.ImportedConfigs, error) {
-        return nil, grpc.Errorf(codes.Unavailable, "not implemented")
+        rev, err := r.RulesCache.ImportConfigs(c)
+        if err != nil {
+                logging.WithError(err).Errorf(c, "Failed to fetch service accounts configs")
+                return nil, grpc.Errorf(codes.Internal, err.Error())
+        }
+        return &admin.ImportedConfigs{Revision: rev}, nil
 }