[GCP/Provisioner] Assign VMs launched on GCP with service account

[Michaelvll]

Fixes #2966
Assign the VMs laucnhed on GCP with service account. This is the original behavior which is missed in our new provisioner implementation.

Tested (run the relevant ones):

• Code formatting: bash format.sh
• Any manual or new tests for this PR (please specify below)
  • normal VM: sky launch -c test-gcp-sa --cloud gcp --cpus 2 'curl -H "Metadata-Flavor: Google" "http://metadata.google.internal/computeMetadata/v1/instance/service-accounts/default/identity?format=standard&audience=gcp"'; check the VM on the console; `
  • multiple nodes: sky launch -c test-gcp-sa-mn --cloud gcp --num-nodes 4 --cpus 2 'curl -H "Metadata-Flavor: Google" "http://metadata.google.internal/computeMetadata/v1/instance/service-accounts/default/identity?format=standard&audience=gcp"'; check the VM on the console`
  • TPU VM: sky launch -c test-gcp-sa-tpu --cloud gcp --gpus tpu-v2-8 'curl -H "Metadata-Flavor: Google" "http://metadata.google.internal/computeMetadata/v1/instance/service-accounts/default/identity?format=standard&audience=gcp"'; check the VM on the console; `
  • multi-node TPU VM: sky launch -c test-gcp-sa-tpu --cloud gcp --gpus tpu-v2-8 --num-nodes 2 'curl -H "Metadata-Flavor: Google" "http://metadata.google.internal/computeMetadata/v1/instance/service-accounts/default/identity?format=standard&audience=gcp"'; check the VM on the console; `
• All smoke tests: pytest tests/test_smoke.py
• Relevant individual smoke tests: pytest tests/test_smoke.py::test_fill_in_the_name
  • pytest tests/test_smoke.py::test_gcp_region_and_service_account
• Backward compatibility tests: bash tests/backward_comaptibility_tests.sh

[concretevitamin]

Thanks for the quick fix @Michaelvll. LGTM.