[GCP] Update credentials filename

[texasmichelle]

TF-serving sets the value of GOOGLE_APPLICATION_CREDENTIALS to the contents of key.json, which is incompatible with the secret as setup by the deploy script (user-gcp-sa.json). This needs to be updated to match the filename used by the automated deploy.

[jlewi]

/assign @lluunn

[sarahmaddox]

Thanks @texasmichelle!
@jlewi This fix is blocking the GKE end-to-end guide (website PR #151). Please can we apply the fix to version 0.2.x as well as 0.3, as I'd like to move forward with the publication of the guide as soon as possible.

[texasmichelle]

I second @sarahmaddox's request to apply the patch to 0.2

[texasmichelle]

#1716 addresses this issue.

Shouldn't there be a bot that relates the PR to the issue? Is that a GitHub feature or repo-specific?