This patch adds notifications when instance
tags changed, as tag is commonly used for
searching and filtering, and projects like
searchlight depends on the notifications.
To avoid unnecessarily building a notification
payload when notifications are disabled, this
change also adds the if_notifications_enabled
decorator to send_instance_update_notification.
Change-Id: I03c8e8225e51fd80580772752c0b292987e34218
Implements: bp send-tag-notification