Check CLOUDSDK_CONFIG for default credentials location

[ajkannan]

In DefaultCredentialsProvider.getWellKnownCredentialsFile(), the environment variable CLOUDSDK_CONFIG isn't checked. Instead, it assumes that the .config directory is in home. If the CLOUDSDK_CONFIG environment variable is set, that should be used instead of home. This would be consistent with the issue reported in the python equivalent of this library (see this issue).

As a result of not checking CLOUDSDK_CONFIG, users can see the issues like this if they install Cloud SDK in a non-default location.

[aozarov]

ping. /cc @anthmgoogle

[anthmgoogle]

@lesv @ejona86. Agree this is a bug that should be fixed.

[lesv]

I'll try to go through all of these issues next week.

[mziccard]

I believe this was fixed in #63