Default credentials for Google Compute Engine (#13577)
- [x] closes #13577
- [x] passed all existing tests for GbqConnector
- [x] passes `git diff upstream/master | flake8 --diff`
- [x] whatsnew entry

## [Current coverage](https://codecov.io/gh/pydata/pandas/pull/13578?src=pr) is **84.30%**

> Merging [#13578](https://codecov.io/gh/pydata/pandas/pull/13578?src=pr) into [master](https://codecov.io/gh/pydata/pandas/branch/master?src=pr) will decrease coverage by **<.01%**

``` diff
@@             master     #13578   diff @@
==========================================
  Files           138        138          
  Lines         51155      51160     +5   
  Methods           0          0          
  Messages          0          0          
  Branches          0          0          
==========================================
  Hits          43130      43130          
- Misses         8025       8030     +5   
  Partials          0          0          
```

> Powered by [Codecov](https://codecov.io?src=pr). Last updated by [2655dae...fd898e3](https://codecov.io/gh/pydata/pandas/compare/2655daef1b7346feabd00d4d40910a80386d0812...fd898e3edd3cd515d1799cec0ad9e18d1569c338?src=pr)
