Support specifying disk image for additional disk with Google Batch (e.g. Image with reference files)

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

Suggest implementation

Update Google Batch Task handler to use disk image if both type and image are specified. This will take advantage of the current code which applies to disk type

This could look like this

I've created a sample implementation in a PR

Start with the Google Batch Task handler and the sample implementation referenced by the issue. Check how disk directives currently handle disk type, then verify that specifying both a disk type and image uses the image for the additional disk; the linked pull request indicates work is already underway.