[GCP] Fix spot with machine image

[Michaelvll]

Fixes #3235

We have some missing configurations for launching spot instances on GCP with machine image, which seems caused by the API update of GCP.

Note: all the configs added are required to make spot work with machine image.

Tested (run the relevant ones):

• Code formatting: bash format.sh
• Any manual or new tests for this PR (please specify below)
  • sky launch -c test-machine-image test.yaml

    resources:
      cloud: gcp
      region: us-west1
      image_id: projects/skypilot-xxxx/global/machineImages/test-machine-image
      cpus: 2+
      memory: 4+
      disk_size: 256
      use_spot: true
    
    num_nodes: 2

  • sky launch -c test-normal test.yaml

    resources:
      cloud: gcp
      region: us-west1
      cpus: 2+
      memory: 4+
      disk_size: 256
      use_spot: true
    
    num_nodes: 2

• All smoke tests: pytest tests/test_smoke.py
• Relevant individual smoke tests: pytest tests/test_smoke.py::test_fill_in_the_name
• Backward compatibility tests: bash tests/backward_comaptibility_tests.sh

[concretevitamin]

Thanks @Michaelvll!