Add option to attach multiple snapshot schedule to disks

[shujaatsscripts]

Currently only one snapshot schedule is allowed to attach to the disk. The variable take a string.

This PR added the option to give the list of snapshot schedules to attach to the disk/boot disk. This option is not allowed through GCP console but its available in terraform and gcloud cli.

Example: If we need to take snapshots both daily and weekly, we need to attach 2 snapshot schedule to one disk.

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Checklist

I applicable, I acknowledge that I have:

• Read the contributing guide
• Ran terraform fmt on all modified files
• Regenerated the relevant README.md files using tools/tfdoc.py
• Made sure all relevant tests pass

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[ludoo]

Thanks for this, very clean PR