Make number of CV folds configurable

[vonHartz]

Hi Marius,

when using the CSV-facade, one can specify the CV-folds with an extra CSV file:

AutoFolio/autofolio/facade/af_csv_facade.py

Line 27 in f296f52

self.scenario.read_from_csv(perf_fn=perf_fn,

However, when not doing that, it will call the ASLibScenario without specifying the number of CV folds:
https://github.com/mlindauer/ASlibScenario/blob/9ce38dd43dc6e2834cb8ef44cd837514e950a9c5/aslib_scenario/aslib_scenario.py#L168
In that case, the default of 10 folds will be used:
https://github.com/mlindauer/ASlibScenario/blob/9ce38dd43dc6e2834cb8ef44cd837514e950a9c5/aslib_scenario/aslib_scenario.py#L827

It would be convenient, if one could, instead of having to create a full CVS specifying the splits, just pass a custom number of CV folds to use AutoFolio more easily with different dataset sizes.

Let me know if you'd like me to provide a pull request.

Best,
Jan