Changes driver method for notifications

Oslo.messaging.Transport is used as an abstraction to redirect function
calls to the configured driver. _send_notification called
self._driver.send instead of self._driver.send_notification

As the test
oslo.messaging.tests.TestTransportMethodArgs.test_send_notification also
assumed that send was the correct method, this should be changed
accordingly

Change-Id: I9406d74f3dc13c44d1aaad5379aafbf1a8580137