Ability to mirror from alternate registry location via registries.conf

Story: As a user of disconnected OpenShift clusters, I want to validate disconnected support for operator prior to General Availability (GA).

Background: 

Request support for alternate registry location to be specified via registries.conf during pre-production disconnected testing/validation. This allows teams to reference the final production location of images always in catalogs, ImageSetConfigs, etc. See https://github.com/openshift/oc/issues/1163

Acceptance criteria:

• oc-mirror will accept registries.conf as input and redirect to registry mirrors as applicable.