Support specifying disk image for additional disk with Google Batch

[hnawar]

New feature

Support specifying a disk image for additional disk when using Google Batch. This would be useful in many cases but particularly with large reference files which can be both time consuming and expensive. Using DIsk image is faster and cheaper as the Disk is created from the image before the provisioning the VM

Usage scenario

User creates a Disk image containing all reference files User specifies the image as part of the disk directive in the process

#4534