Pass serializer to oslo.messaging Notifier

oslo.messaging has a workaround [1] that requires context to be
transformed to pure dict before passing into amqpdriver.

Renamed serializer class to reflect its broader usage.

Updated FakeNotifier to expect serializer and other keyword arguments
supported by oslo.messaging Notifier class.

[1]: oslo/messaging/_drivers/amqpdriver.py#L337

blueprint oslo-messaging

Closes-Bug: #1332412
Change-Id: I7e7658d03639afae7bf6d3ad71445cb5b6459c09
Merge "Pass serializer to oslo.messaging Notifier"