Provide a ranking mechanism for glance-api to order locations

We have a setup consisting of multiple cells with their own glance-api servers. Currently, it is not possible for each cell to specify their custom ordering of image locations. If this is made possible, each cell can return the closest location for each glance image, and also do copy-on-write for stores like RBD. This can be achieved by having another location_strategy that orders based on a ranking key in metadata.