[Test] Fix test_kubernetes_show_gpus for any k8s clusters

[kevinmingtarja]

Previously, we were hardcoding the GPU count to be 8/8. But this is only true for the kind clusters in our test infra. This PR fixes it to use a regex pattern for matching, so that it can run on arbitrary k8s clusters with arbitrary amount of GPUs (or no GPUs too).

Tested (run the relevant ones):

• Code formatting: install pre-commit (auto-check on commit) or bash format.sh
• Any manual or new tests for this PR (please specify below)
• All smoke tests: /smoke-test (CI) or pytest tests/test_smoke.py (local)
• Relevant individual tests: /smoke-test -k test_name (CI) or pytest tests/test_smoke.py::test_name (local)
• Backward compatibility: /quicktest-core (CI) or pytest tests/smoke_tests/test_backward_compat.py (local)

[kevinmingtarja]

/smoke-test --kubernetes -k test_kubernetes_show_gpus

[kevinmingtarja]

/smoke-test --kubernetes -k test_kubernetes_show_gpus --env-file s3://buildkite-configfile-test/test_gke_config_1.yaml 🟢 now (Failed before this PR)

[SeungjinYang]

Thanks @kevinmingtarja for making the test more flexible!