[GCP] fix application key upload

[Michaelvll]

Our previous way for ignoring the application key is problemetic, as we will raise a FileNotFoundError when calling self._find_application_key_path() and the key file is not found, i.e., the later if os.path.exists(os.path.expanduser(application_key_path)): does not help for ignoring the credential file.

Tested (run the relevant ones):

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

[cblmemo]

Thanks! LGTM.