Support Upload Image Data To The Specified Backend In V2

Now,glance v2 doesn't support upload the image data to the specified backend. It means that users could only upload the image data the default glance store backend even there are some backend support for glance.

In v1, users could pass a header named "x-image-meta-store" to indicate which backend they want the image data be stored.

In v2, I suggest that we can introduce a new optional parameter to image date upload API. It could be named "data_store" or something else. It will be used for let users decide which backend the image data will be saved. If it dosen't be passed. The image data will be saved to the default backend which is contained in config file.

Or we could do it the same as v1, to use "x-image-meta-store".