add validate user's token method for v2 and bug fix for reauth
diff --git a/openstack/client.go b/openstack/client.go
index 1193b19..62855f0 100644
--- a/openstack/client.go
+++ b/openstack/client.go
@@ -167,6 +167,7 @@
 
 	if options.AllowReauth {
 		client.ReauthFunc = func() error {
+			client.TokenID = ""
 			return AuthenticateV3(client, options)
 		}
 	}