[GCP/Auth] Use the application key in the env var

[Michaelvll]

Fixes #2115 and #2109.

The problem causing the two issues is that we hardcoded the application default key path in our code to find the credentials. However, the user may specify the key in the environment variable GOOGLE_APPLICATION_CREDENTIALS

We now first check the env var to look for the credential file, if the env var does not exist, we will use the default path (same as the gcloud's behavior).

TODO:

• add instruction for log in with service account.

Tested (run the relevant ones):

• Code formatting: bash format.sh
• Any manual or new tests for this PR (please specify below)

  Download the service account key
  mv ~/.config/gcloud ~/.config/gcloud.bk
  export GOOGLE_APPLICATION_CREDENTIALS="path/to/the/key/file"
  gcloud auth activate-service-account --key-file $GOOGLE_APPLICATION_CREDENTIALS
  gcloud config set project project-id
  sky launch -c test-sa --cloud gcp --cpus 2 -i 0
  sleep 120
  sky status -r test-sa
  

• All smoke tests: pytest tests/test_smoke.py with the service account. (except for TPU VM and azure_start_stop [Core] Avoid uploading public key #2119)
• Relevant individual smoke tests: pytest tests/test_smoke.py::test_fill_in_the_name
• Backward compatibility tests: bash tests/backward_comaptibility_tests.sh

[concretevitamin]

LGTM, thanks @Michaelvll.