Use driver's notify_send() method again

Drivers have a notify_send() method which allows for special semantics
to be implemented for notification sending. During the port over to
oslo.messaging, it appears we stopped using this and switched to just
using topic_send() instead.

Lets restore the use of notify_send() so that we don't lose behaviours
like I9f8dfd6eaf3996be58fecff6ad91508bdcef23f3 added.

Change-Id: Id0ad0431d3e2b1bfe21723e7f3b1842c2d3a9546
Merge "Use driver's notify_send() method again"