Adding image multiple location support

[OpenStack Infra]

https://review.openstack.org/#/c/33409/
commit 6dc1e3b3f72d06f8095d42e8d2b726922f952f73
Author: Zhi Yan Liu <email address hidden>
Date: Sat Jun 15 21:25:34 2013 +0800

    Adding image multiple location support

    Image now support multiple locations within its metadata and it maybe
    stored more then one backend storage. Nova should add a layer to
    transparent handle image preparing and removing for an instance by the
    best approach/location, and it should allow administrator to configure
    the image handler pipline with the order who preferred to the layer.

    Also, base on this structure we could implement particular sub-class in
    relevant hypervisor layer with more advanced functions, such as COW
    making, snapshot capturing and etc. in feature.

    Implement bp: image-multiple-location
    docimpact

    Change-Id: Idce8d21ae37bfdbb28a2567120a83d1061061904
    Signed-off-by: Zhi Yan Liu <email address hidden>