The make task for installing required Operators fail

make install_required_operators fails due to changes that were introduced in !172 (merged). This is a documented step for installing Operator.

Here is the error message:

error validating "scripts/manifests/cert-manager-values.yaml": error validating data: ... 
decode base64: illegal base64 data ... 
"data":{"credentials.json":"GOOGLE_CREDENTIALS"},

The problem is that GOOGLE_CREDENTIALS is now required for this task particular task to work and it doesn't work on existing clusters.

We either need to remove this step from the documentation or ensure that it will not fail on non-GKE and existing clusters.