Comment 1 for bug 1387441

add a `last_modified` field to the pool, when update the pool, this field is updated.
Then when the catalog get the drivers[0]. Compare the the `last_modified` field with the value cached in the catalog.

* if they are equal, the do noting and use the cached driver.
* if they are not equal, then destroy the old one and init a new driver by using the lasted pool options.

Is this a good direction?

[0] https://github.com/openstack/zaqar/blob/master/zaqar/storage/pooling.py#L530