[LIBCLOUD-537] Added support for generic image management at Rackspace and EC2

I was planning to take care of GCE in this first push as well. However, Google presents an interesting challenge with their persistent disk model and their use of the snapshot and image terms. We could use snapshots as a replacement for images. However, this would then limit our ability to use the default images provided by Google. It seems like we actually need a factory for evaluating both the images and snapshots in order to fold in GCE. I completed the _supported_methods_image_management.rst as well in case we want to include it in the documentation. I did submit an email regarding this effort to the dev mail list but I didn't hear anything back so I just went ahead and made the change.