Proactive Nova Compute Image Caching

Extending Nova in order to enable a wider control over image caching on the compute nodes. The main goal of this extension is to allow a specific compute node to pre-cache VM-images on command. Given an image I and a node N, the extension will allow an administrator to:
-> Trigger the download of I on N, and keep it into the node.
-> Override the default behavior of the cache-cleaner (if not used during the last day, it is deleted).
-> List all the cached images on node N.
-> Delete the image I from the cached ones on the node N.