Change GBT validationTol to be relative tolerance

In spark.mllib, GBT validationTol uses absolute tolerance. Relative tolerance is arguably easier to set in a meaningful way. Questions:

• Should we change spark.mllib's validationTol meaning?
• Should we use relative tolerance in spark.ml's GBT (once we add validation support)?

I would vote for changing both to relative tolerance, where the tolerance is relative to the current loss on the training set.