Allow users to select S3 Reduced Redundancy Storage

Amazon offers a reduced redundancy option to allow users control their costs by sacrificing some reliability.

The boto s3 client supports this option, so the glance add_image API should allow users to specify s3-specific image properties that cause the option to be selected.

One option would be to apply this QoS across-the-board for all images stored in S3
if a global config option is set, however it would usually depend on the nature of the image as to whether RSS is appropriate (e.g. custom images or snapshots are less reproducible and hence should be stored with maximum redundancy).

So the other option would be to support a generic header to allow the user to hint via a generic header (e.g. x-glance-reduced-redundancy) that reduced storage redundancy is acceptable for a particular image (with the caveat that this hint may have no effect if the underlying store only provides a single level of redundancy, as does Swift).