Add attribute for notification_driver

- Add default ['openstack']['block-storage']['notification_driver'] attributes
- Value defaults to the same as before, so no change, but attribute can
  be overridden.
- Add test for the attribute
- Add README.md entry for the attribute

Change-Id: I3ee6f044a0d9d7a46f5e0225ae440fac4f9255a3
Closes-Bug: #1275714