RFE: less hard coding of UUIDs

[jim-minter]

I think it'd make it easier on users of osb-checker if it were necessary to customise test/configs/config_mock.json less - e.g. if the framework could autodiscover and use the sole plan_id of a service_id, and/or the sole service_id in a catalog, etc.

Clearly this needs to be done carefully to allow for advanced users, but some sort of quick-start functionality would be pretty handy - both for new users and setting up test automation.