Remove gridable=false for certain parameters in DeepLearning and GBM/DRF (balance_classes).

I had misunderstood the meaning of "gridable=false": Thought (wrongly) that
this would disallow the parameter from being "grid-searched over", (i.e.,
specification of multiple values), while it actually just omits the value from
being passed to the individual grid search jobs (i.e., the default will be
used).
Merge branch 'master' of https://github.com/0xdata/h2o