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. 
Sounds good to me. I was wondering how to handle these responses, I noticed that some similar methods did not have a fallback such as Disk:insert but then others did such as Disks:createSnapshot. I see that is addressed in PR#40.

I will remove the @Fallback(NullOnNotFoundOr404.class) line and the corresponding expect test 
testCreateImageFromPdResponseIs4xx().