Add the support of block level backup

So far in Cinder, the volume backup is implemented via reading the data of the volume and metadata, and saving them into an object in a object storage backend, such as swift.
This blueprint can add the support of backuping a volume and its metadata in another destination volume. At the first step,
we can create a backup volume, attach it to the same host as the original volume and then do the "dd" or "dump" command.
In future, it would be ideal to consider the rsync over ssh between two hosts.