Remove hard coded images from go source files

[scothis]

Expected Behavior

Knative Build should only consume images declared in its configuration. This enables tools to scan what images will be consumed, and provide alternate images as needed

Actual Behavior

The gcr.io/cloud-builders/gcs-fetcher:latest image is defined in the source code. The other image arguments have stub values with actual values defined in the controller deployment. Because the image is not also specified in the yaml configuration, it requires special foreknowledge of Knative Build in order to properly configure the controller.

Additional Info

build/pkg/reconciler/build/resources/pod.go

Line 103 in 7e63198

gcsFetcherImage = flag.String("gcs-fetcher-image", "gcr.io/cloud-builders/gcs-fetcher:latest",