Split drivers:storage into (message|management_storage)

[Flavio Percoco]

Turns out we forgot to split the option under `drivers`, which makes it impossible to use 2 different store types - 1 for mgmt and 1 for messages - in Zaqar.

[OpenStack Infra (hudson-openstack)]

Fix proposed to branch: master
Review: https://review.openstack.org/188541

[OpenStack Infra (hudson-openstack)]

Reviewed: https://review.openstack.org/188541
Committed: https://git.openstack.org/cgit/openstack/zaqar/commit/?id=de1b3307fa0ffd9f15a185816093fa575ae4c327
Submitter: Jenkins
Branch: master

commit de1b3307fa0ffd9f15a185816093fa575ae4c327
Author: Flavio Percoco <email address hidden>
Date: Thu Jun 4 20:18:57 2015 +0200

    Split `storage` into (message|management)_storage

    We split both, data and control, planes and we were diligent in
    splitting the configuration sections. However, we forgot to split the
    `storage` option in the `drivers` section.

    This patch does that and marks the old option as deprecated.

    Closes-bug: #1462025
    Change-Id: I692c35dcf217bede557ac463b2388f9a60d96c79