Move LoginHandler into LoginServer which others embed (#3431)

* Move LoginHandler into LoginServer which others embed

* Add unit tests for loginServer

cmd/storage-rpc-server.go

@@ -29,32 +29,12 @@ import (
 // Storage server implements rpc primitives to facilitate exporting a
 // disk over a network.
 type storageServer struct {
+	loginServer
 	storage   StorageAPI
 	path      string
 	timestamp time.Time
 }
 
-/// Auth operations
-
-// Login - login handler.
-func (s *storageServer) LoginHandler(args *RPCLoginArgs, reply *RPCLoginReply) error {
-	jwt, err := newJWT(defaultInterNodeJWTExpiry, serverConfig.GetCredential())
-	if err != nil {
-		return err
-	}
-	if err = jwt.Authenticate(args.Username, args.Password); err != nil {
-		return err
-	}
-	token, err := jwt.GenerateToken(args.Username)
-	if err != nil {
-		return err
-	}
-	reply.Token = token
-	reply.Timestamp = time.Now().UTC()
-	reply.ServerVersion = Version
-	return nil
-}
-
 /// Storage operations handlers.
 
 // DiskInfoHandler - disk info handler is rpc wrapper for DiskInfo operation.