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. 
I don't think we can get rid of it because in the tests below I need the disk to create an image from.