Port to oslo.messaging.Notifier API

Add a temporary nova.notifier.Notifier helper class which translates
oslo.messaging.Notifier compatible calls into openstack.common.notifier
compatible calls.

This allows us to port the notifier code over to the oslo.messaging API
before actually switching over oslo.messaging fully.

This patch contains no functional changes at all, except that all
notifications go through this temporary helper class.

Some notes on the new API:

  * The notifier API is changed so that what was previously global state
    is now encapsulated in a Notifier object. This object also includes
    the publisher_id and has error()/info()/etc. methods rather than
    just notify().

  * The notify_decorator() helper wasn't carried across to the new API
    because its semantics are a bit weird. Something along these lines
    could be added in future, though.

  * We use a fake Notifier implementation for tests because there's no
    API in oslo.messaging to actually get the notifications queued
    up in the fake notification driver, which is a bit dumb. However,
    this feels like the right thing to do anyway. We're not wanting
    to test oslo.messaging.Notifier itself, but rather we want to test
    how we call it.

blueprint: oslo-messaging
Change-Id: I262163c7e05e6a6fb79265e904ce761fc3ac5806
Merge "Port to oslo.messaging.Notifier API"