out_stackdriver: Don't overwrite ctx->client_email and ctx->private_key [backport to 2.1]

[igorpeshansky]

Backport of #8093 to the 2.1 branch.

This fixes the use case of specifying the service_account_email and service_account_secret options explicitly.

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Testing
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• Example configuration file for the change

  [OUTPUT]
    Name        stackdriver
    Match       …
    service_account_email …
    service_account_secret …
  

• [N/A] Debug log output from testing the change

• Attached Valgrind output that shows no leaks or memory corruption was found

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Backporting

• Backport to latest stable release.
  This is the backport.

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