Migrate GKE cluster off basic authentication

GKE v1.9+ no longer supports basic authentication. [1] To continue receiving automatic updates to the GKE cluster, it must be migrated off of basic authentication, to a supported alternative.

Documentation should also be refreshed to describe the current configuration of the clusters.

[1] https://cloud.google.com/kubernetes-engine/docs/how-to/api-server-authentication#legacy-auth

Clusters:
io-datastores
metrics
metrics-staging