Respect existing cluster pull secret configuration

[everettraven]

Catalogd currently requires pull secrets to be passed through the Catalog resource's spec.source.image.pullSecret field. The Secret referenced in that field must be in the same namespace as catalogd is deployed in (typically catalogd-system). Catalogd should be able to identify any existing pull secrets and automatically resolve them for a registry that is referenced in the spec.source.image.ref field.