Support multiple stores of Glance

[OpenStack Infra]

https://review.opendev.org/661676
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commit 350973f3dddd8e3b1d521238c69a5bd8c8df3fec
Author: Abhishek Kekane <email address hidden>
Date: Thu May 16 08:53:29 2019 +0000

    Support multiple stores of Glance

    Glance now has ability to configure multiple stores at a
    time. To use this feature in cinder for uploading volume to image, operator
    need to define a new field named 'image_service:store_id' in the
    volume-type extra-specs. At a time of volume upload to image request, if
    'image_service:store_id' is present in the associated volume type, then
    image will be uploaded to specified 'store_id'. The value 'store_id' is
    nothing but store identifier defined in glance-api.conf. If the value
    of 'image_service:store_id' is null or not set in volume-type then the
    image will be uploaded to default store in glance.

    Co-authored-by: Sagar Waghmare <email address hidden>
    Co-authored-by: Abhishek Kekane <email address hidden>

    DocImpact
    Implements: bp support-glance-multiple-backend

    Change-Id: Ica56833a1d9fb9f47b922dbbc6558901bb3a2800