Make google-auth an optional dependency

**What is the feature and why do you need it**:

`google-auth` is a required dependency which seems to only be used for kubernetes running on GCP.

**Describe the solution you'd like to see**:

When installing kubernetes (`pip install -U kubernetes`), `google-auth` does not get installed.

However, `google-auth` can be installed as an optional dependency: `pip install -U kubernetes[gcp]`
