Migrate to oslo.messaging

Glance currently uses a custom notifier and it has been maintaining it
for a long time. In a hope of reducing duplicated code and improving
cross-project contributions, this patch replaces the old notifier with
the one, recently developed, in oslo.messaging.

The oslo.messaging project is a port of the old oslo-rpc code to a
standalone, more stable and improved project. It brings all the benefits
that oslo-rpc would've brought as well as an easier way to integrate
with other projects.

This patch also:

    - Reduces the code shipped along with Glance since all the code
      copied from oslo-incubator related to the notifier is not needed
      anymore.
    - Improves the stability of existing, broker based, notifications.
    - Brings HA support.
    - Keeps backward compatibility by translating the old
      `notifier_strategy` into oslo.messaging drivers.

Changes to the code:

    - It is now necessary to pass the request context to the
      notification call.
    - Notifier package is no longer necessary. A notifier module was
      added instead.
    - New, notifier related, configurations were added.
    - A lot of code was removed

Since there's still not an official release, requirements.txt points to
the latest tarball created. A release for oslo.messaging is planned for
Icehouse.

docImpact
Implements bp oslo-messaging

Change-Id: I8cd84772bc5867e06b2a50ed7e15b9e86f0b94ad
Merge "Migrate to oslo.messaging"