The backend driver should be reloaded after the pool is updated

[Jeffrey Zhang]

After updating the pool's option, the backend driver behind pool should be reload to take these new options.

[Jeffrey Zhang (jeffrey4l)]

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

[Jeffrey Zhang (jeffrey4l)]

Another question:
I notice that we are using sqlalchemy without any db migration tool. How can we add new fields in sqlalchemy? It is not very friendly to produce environment. Should we add the migration tools?

[OpenStack Infra (hudson-openstack)]

Fix proposed to branch: master
Review: https://review.openstack.org/143857

[OpenStack Infra (hudson-openstack)]

Change abandoned by Jeffrey Zhang (<email address hidden>) on branch: master
Review: https://review.openstack.org/143857