Problem: can't add non image disk on GCP

[spheromak]

Solution: Only the root disk on GCP has an image requirement. You can
attach other block storage without a source image, and this change
allows for that.

[CLAassistant]

All committers have signed the CLA.

[hardikdr]

Thanks for the PR.
Generally we expect that application instance would request a non-boot disk via PVC/PV mechanism and MCM would not create any in-parallel to boot-disk.
I am curious if you have any specific use-case for this.[eg. local-storage]

Though PR looks good to me, just a side-note would be that extra non-boot disks could potentially become orphan if VM is lost and AutoDelete is not set to true. Essentially there is no way for MCM/Kubernetes to garbage-collect those Disks due to lack of metadata.