JCLOUDS-649: Added image creation from pd, changed rawDisk to Optional<T>

Users can now create images from persistent disks. 

When images are made from PD the rawDisk attribute is not present. 
This changes Image.java to treat it as an Optional<RawDisk> instead. 
This kind of fallbacks shouldn't be used in PUT or POST operations, as the returned type won't be clear (see https://github.com/jclouds/jclouds-labs-google/pull/40). *Write* operations on a resource should assume that the resource already exists or fail otherwise. Using this fallback would silently return `null`, which has no clear meaning to the user: did the operation fail? Did it complete and the response had no body? Others?

Just remove the fallback, and a `ResourceNotFoundException` will be automatically thrown by jclouds if a 404 response is returned.