add support for pd-balanced root_volume_type with spotinst_ocean_gke_launch_spec

[BigSammich]

Description

Currently spotinst/ocean-gcp-k8s-vng/spotinst only supports pd-standard and pd-ssd. This request would be to add pd-balanced as an additionally supported value for root_volume_type.

Presumably, pd-extreme would also need to be supported.

References

https://cloud.google.com/compute/docs/disks#disk-types

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