Enable plugin to work with secure migration registry

[sseago]

cloned fusor/ocp-velero-plugin#50 to new repo

The current implementation of the (temporary) migration registry creates a registry pod which doesn't use https or require login credentials. Once we implement migtools/mig-controller#144 in the controller, the plugin needs to be updated to support obtaining certs and credentials created by the controller (probably via secrets created in the velero namespace with some supporting annotations on the velero CR) and use them to access the registry.