[GCP] Fix machine image for cluster creation

[Michaelvll]

Fixes #3027

The bulkInsert API fails to create VMs using machineImage along with disk parameter override, as it does a check for the source of the disk, which should not be applied when machineImage is specified (bug of GCP API).
This PR makes it fallback to the regular insert API instead to support the creation of instances using 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-image2 --cloud gcp --disk-size 256 --num-nodes 2 --gpus A100-80GB:8; correctly failover through multiple regions
  • sky launch -c test-machine-image5 --cloud gcp --image-id p rojects/skypilot-375900/global/machineImages/test-machine-image --disk-size 256 --cpus 2 --num-nodes 4, successfully create the cluster
  • sky launch -c test-machine-image5 --cloud gcp --image-id projects/skypilot-375900/global/machineImages/test-machine-image --disk-size 256 --num-nodes 2 --gpus A100-80GB:8 successfuly failver through zones/regions.
• All smoke tests: pytest tests/test_smoke.py --gcp
• Relevant individual smoke tests: pytest tests/test_smoke.py::test_fill_in_the_name
• Backward compatibility tests: bash tests/backward_comaptibility_tests.sh

[cblmemo]

Thanks for the fix! Left several comments 🫡

[cblmemo]

LGTM. A nit: should we modify the function name here to sth like _provision? Though I'm happy with the current implementation since most of the time it won't trigger single inserts.

skypilot/sky/provision/provisioner.py

Line 53 in 16cdc7f

def _bulk_provision(

[Michaelvll]

LGTM. A nit: should we modify the function name here to sth like _provision? Though I'm happy with the current implementation since most of the time it won't trigger single inserts.

skypilot/sky/provision/provisioner.py

Line 53 in 16cdc7f

def _bulk_provision(

Thanks for the suggestion @cblmemo! I suppose the _bulk_provision is more about provisioning multiple instances in a single function, it might be fine to keep it this way. : )