To be able to separate the versioned notification related
object model from the nova internal object model a new
nova.notification.object package is introduced.
As nova had nova/notifications.py already in the tree
the code from that file is moved into the new nova.notification
package as a first step.
Implements: bp versioned-notification-transformation-newton
Change-Id: I315f548180c260c22a2ebcf7b7cb56d284b6e580