Allow loading CB model with CCB default arguments

[slahabar]

CB model and CCB models trained with 1 slot will be equivalent.
VowpalWabbit/vowpal_wabbit#2610

And also, since CCB model is a superset of CB model, we should allow using CB model with CCB default arguments.
Required by Personalizer for adding multi-slot CCB algorithm.

https://microsoft.sharepoint.com/:w:/t/personalizr/EdkGA9DfsTJDhh5zv0i0zk0BNvpmBRXyG9Pg37wpA6iSRQ?e=cPaQFa&%7BclickTime%7D=1601572667849