Added default auth & token URLs instead from the wellknownfile

[spothala]

• 2LO flow is not working with ~/.config/gcloud/application_default_credentials.json as it doenst contain fields token_uri & auth_uri
• So pointing them to default URLs

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[googlebot]

Thanks for your pull request. It looks like this may be your first contribution to a Google open source project (if not, look below for help). Before we can look at your pull request, you'll need to sign a Contributor License Agreement (CLA).

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Corporate signers

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[googlebot]

CLAs look good, thanks!

[googlebot]

CLAs look good, thanks!

[shinfan]

We should not use hardcoded urls here. How about using the default urls as fallback when the file url is empty?

[spothala]

Thats totally makes sense. I assumed ADC file wont have any auth & token urls. Let me update the change accordingly.

[wora]

The default URL has changed. The new one is oauth2.googleapis.com/token. Please fix it.

[shinfan]

#50 ready for review