Gke optional accelerator

[ankitkinra]

Make GKE guest_accelerator_config truly optional

With this fix following nodepool configuration will be supported, where type/count are picked up from g2-standard-4 but the driver is picked from configuration

- id: g2_pool
    source: community/modules/compute/gke-node-pool
    use: [gke_cluster]
    settings:
      name: g2-latest-driver
      disk_type: pd-balanced
      machine_type: g2-standard-4
      guest_accelerator:
      - gpu_driver_installation_config:
        - gpu_driver_version: "LATEST"

In the below configuration for n1_pool where the gpu_driver_version is defaulted to DEFAULT

  - id: n1_pool_defaults
    source: community/modules/compute/gke-node-pool
    use: [gke_cluster]
    settings:
      name: n1-pool-defaults
      disk_type: pd-balanced
      machine_type: n1-standard-1
      guest_accelerator:
      - type: nvidia-tesla-t4
        count: 2