IOVDBSvcConfig: auto-configure CREST backend based on GlobalTag

We don't have a good way to configure the IOVDbSvc.Source property in CA configurations. But rather than adding yet another flag, I propose we auto-configure the CREST backend if the GlobalTag starts with "CREST-".

But I am open for other suggestions in case this doesn't cover all use-cases.

cc @tsulaia @wlampl @sroe @formica