Add multi-store support

[OpenStack Infra]

https://review.openstack.org/574582
Dear bug triager. This bug was created since a commit was marked with DOCIMPACT.
Your project "openstack/glance" is set up so that we directly report the documentation bugs against it. If this needs changing, the docimpact-group option needs to be added for the project. You can ask the OpenStack infra team (#openstack-infra on freenode) for help if you need to.

commit cb45edf5c81f7d09c9ef0b88d40d56b4750beb10
Author: Abhishek Kekane <email address hidden>
Date: Mon May 7 10:30:01 2018 +0000

    Add multi-store support

    Made provision for multi-store support. Added new config option
    'enabled_backends' which will be a comma separated Key:Value pair
    of store identifier and store type.

    DocImpact
    Depends-On: https://review.openstack.org/573648
    Implements: blueprint multi-store

    Change-Id: I9cfa066bdce51619a78ce86a8b1f1f8d05e5bfb6

[Brian Rosmaita (brian-rosmaita)]

https://review.openstack.org/#/c/576075/ was included in Rocky RC-1