Change notifier_strategy to notification_driver

The notifier_strategy config option is deprecated in favour of
notification_driver in Icehouse.  This change updates the configuration
while also changing attribute namespace since the option is not
actually mq-specific.

Once this change has been accepted we can remove the
default['openstack']['mq']['image']['notifier_strategy'] attribute in
the common cookbook.

Change-Id: Iad09ec0c7fe2f66ab084c363173a98f2737db146
Closes-Bug: #1303784