Sourcegraph.com should use *_CONFIG_FILE via Kubernetes

Created by: slimsag

We can now commit our Sourcegraph.com configuration to our repository using the new *_CONFIG_FILE environment variables. We should also set DISABLE_CONFIG_UPDATES after https://github.com/sourcegraph/sourcegraph/pull/4154 is merged.

This would prevent issues like https://github.com/sourcegraph/sourcegraph/issues/3966#issuecomment-494602066 from happening again by accident.