Fix parsing of AMQP configuration

The deprecated notifier_strategy option is always used even if it is set
to its default value and the new notification_driver option is
explicitly set.

This change makes the notification_driver option work.

Change-Id: Icc2a063a828472132d2e2eda7f12e265db990ea1
Closes-Bug: #1271724
Merge "Fix parsing of AMQP configuration"