[datafeed parser] factor out retrieve_url method from DataFeedXMLParser.parse Related to #3682069

[datafeed] Allow to override use_cwuri_as_url in configuration of a datafeed source This is necessary because in some case one will want to write parser which create entities not one-to-one related to entities from the distant source (for instance see the `vcrs` cube which create entities holding statistics for code review, while not importing review entities themselves). Ideally we should provide a way to the parser to indicate the source that cwuri should not be used, but that's a start... Closes #3670210

merge from 3.19 branch

[entity/rset] replace scary warnings about ambiguous rdefs by XXX in the code users can't do anything about that and shouldn't see those warnings.