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

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[bentsherman]

This change by itself is fine, but the disk directive now has three purposes (resize boot disk, attach empty disk, attach disk with image) but can be specified only once. That means that e.g. you couldn't attach a disk with reference inputs and also attack a local SSD to use as scratch storage.

To solve this issue we just need to allow the disk directive to be specified multiple times. But I am fine with merging this PR as it is and implementing multiple disk directives in a separate PR.

[hnawar]

Thanks Ben :)
Note, that the disk can still be used as scratch space if it is not completely full and is mounted in the same path with read/write permissions

I did think about the multiple disk directive, but did not have an answer for that