Add setDiskAutoDelete to InstanceApi

Converts AttachDiskOptions to AutoValue.
@adriancole does it make sense for AttachDiskOptions to be done with AutoValue? I'm still trying to get used to it. 

Also I'm trying to understand the interplay of  NewInstance.Disk, AttachDiskOptions, and Instance.AttachedDisk. They seem redundant and Im wondering if that is  intentional?
That is what I was thinking. I don't feel strongly about it, I just got tired of having to delete things if the tests failed partway through.