Adding image multiple location support

Images now support multiple locations within its metadata and it may be
stored on more than one backend storage. Nova should add a layer to
transparently handle image preparing and removing for an instance by the
best approach/location, and it should allow administrators to configure
the image handler pipeline with the order who preferred to the layer.

Also, based 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 future.

Implement bp: image-multiple-location
DocImpact

Change-Id: Idce8d21ae37bfdbb28a2567120a83d1061061904
Signed-off-by: Zhi Yan Liu <zhiyanl@cn.ibm.com>
Merge "Adding image multiple location support"