U/salnikov/dm 621

In order to ease debugging, it would be convenient to be able to easilly run some operations independently. CSS metadata creation is a good example: if it fails during integration tests, it is convenient for the developer to be able to run only it with the same parameters in irder to focus easilly on the problem. May some options be added in order to be able to run only some complex part of dataloading algorithm? If yes, this can be done in a new ticket.