Add way to customize when cross validation is used / easier way to use cross validation with AutoML

### System information
AutoML 0.17.4

### Issue

After looking through the code it appears that cross validation will be used in AutoML IF you have less than 15,000 rows and you don't provide a custom validation set. I would make a couple of recommendations:

1) Document the 15,000 and the 10 fold somewhere beyond just in the source code. It was more difficult than it should be to find those 2 numbers
2) Allow the 15,000 and 10 fold to be customized. I realize that the 10 fold can be changed with an override of the execute method. However it then returns a CrossValidationExperimentResult which requires extensive custom code to retrieve scores averaged over the folds vs. when AutoML automatically chooses to utilize cross validation and generates summary ExperimentResult