Add serializer capability to fake_notifier.

With the change to oslo.messaging.notify.Notifier, we have
introduced the serializer plugin.  In some implementations
this is used to modify the notfications/serialize data.  The
implementations will want to test their serializers and
the easiest way to do this is via the fake_notifier.
This change allows fake_notifier to act more like
notifier with serializer being a pass in as an argument.

The serializer is passed to subnotifiers created via
the prepare method.

Change-Id: I03ef9ed92366961d4e296381d149c54d7da97f70
Merge "Add serializer capability to fake_notifier."