Simplify public symbol exports

Inspired by tulip, have every module define a __all__ list and import *
from the top-level module.

Rename transport.set_defaults() since we don't want this to be a
top-level set_defaults() function as there may be multiple.

Also, rather than configuring flake8 to allow star imports, just exclude
the __init__.py files from flake8 checks.
Remove backwards compat entry point aliases

There's no point in supporting

  oslo.rpc.impl_kombu

since what we need to support is

  nova.openstack.common.rpc.impl_kombu
Fix notify drivers namespace
Docstrings for notifier API
Make Notifier public at top-level
Rename context to ctxt in notify API

ctxt is the name we've used in the rest of oslo.messaging, so ...
Rename context to ctxt in serializer API
Add serializer support to notifier

The notifier in oslo-incubator does:

  payload = jsonutils.to_primitive(payload, convert_instances=True)

Using the serializer abstraction should be a more general was of
supporting this.
Require a transport to construct a Notifier

Rather than construct it from cfg.CONF by default, just require that one
is supplied.
Make LOG private in notifier
Fix buglet in v2 messaging notifier
Handle case where no drivers configured
Use lowercase priority in notifications topic
Use lowercase priority in log notifier
Make test notifier useful

We need a way to query what notifications were sent to the test notifier
and later clear out that list.
Unit tests for notifier
Add some notes