fs.gdrive: Fix GDRIVE_CREDENTIALS_DATA for service accounts

[RCdeWit]

_validate_config() had a bug: it would not take into account that Google
Drive credentials can also be passed using GDRIVE_CREDENTIALS_DATA as
documented here 1.

This patches the validation logic to also accept this environment variable as a
means to provide service account credentials.

Before, commands like dvc push would exit with:

ERROR: failed to push data to the cloud - To use service account, set
  `gdrive_service_account_json_file_path`, and
  optionally`gdrive_service_account_user_email` in DVC config
Learn more at <https://man.dvc.org/remote/modify>

Thanks to @duijf for helping me diagnose.

Co-authored-by: Laurens Duijvesteijn [email protected]

[pmrowla]

Related: iterative/dvc-gdrive#20

[skshetry]

I think we can just leave this message out.

[dberenbaum]

@RCdeWit Thanks for the contribution! Would you be able to respond to update the PR to address the comments?

[RCdeWit]

Thanks for the review, folks. Incorporated the feedback!

[daavoo]

Not saying it should block the P.R. but we should probably test for this (maybe in https://github.com/iterative/dvc/blob/30d6839a5f471fb02a7a2dcbd6a6616b71277762/tests/unit/remote/test_gdrive.py)

[RCdeWit]

I'm working on part 2 of a blog post that covers this use case. I was hoping to release it somewhere over the next two weeks; any chance this fix will be merged by then? If not, my blog post will have to include the workaround.

# GDRIVE_CREDENTIALS_DATA is not read automatically when using a service account
# Manually write it to creds.json and read that file
echo -E '${{ secrets.GDRIVE_CREDENTIALS_DATA }}' > creds.json

dvc remote add -d -f myremote gdrive://${{ secrets.GOOGLE_DRIVE_URI }}
dvc remote modify myremote gdrive_use_service_account true
dvc remote modify myremote --local gdrive_service_account_json_file_path creds.json

dvc push

# Either do this or add the file to your .gitignore
# Just make sure not to push it to your repository
rm creds.json