Query cloud specific env vars in task setup

[hemildesai]

Adds a class method to Cloud called query_env_vars which can query cloud specific env vars. Addresses #2287

Tested (run the relevant ones):

• Code formatting: bash format.sh
• Any manual or new tests for this PR (please specify below)
  • Ran sky launch -- printenv and verified that KUBERNETES_SERVICE_PORT etc are included
• 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

[romilbhardwaj]

Thanks @hemildesai! This PR got closed when we merged #2096 into master.

For our next sprint, we'll be switching to k8s_cloud_beta1 branch. Can you re-open this PR on that branch?

[hemildesai]

Couldn't reopen this so created #2347