Accomodate airgapped mirror registry

[ron1]

Currently, the mirror registry host seems to require internet connectivity. This may not be the case in a true airgap environment where the registry images and configuration manifests must be mirrored to a directory on removable media. Then, on the internal network, the local images on the removable media must be uploaded to the mirror registry.

Consider supporting the use case where the mirror registry on the internal network is completely disconnected from the internet and must be loaded from removable media.

The oc adm release mirror --to-dir= and oc image mirror --from-dir= functionality described here seems like it could be used to solve this problem. WDYT?