[MRG] Uniformize max_features semantics for extra trees and random forest

The goal of this pr is to uniformize the semantics of max_features:
- random forest: currently draws features until it found a non constant features, then it begins to count the number of features drawn
- extra trees: currently draws `max_features` non constant features

This pr uniformize all behaviours as discussed in #2813: we draw `max_feature` and at least one must be  valid (i.e. non constant).

Benchmarks on covertype:
```
this pr   train-time test-time error-rate
--------------------------------------------
RandomForest  70.6019s   0.3386s     0.0344  (max_features=sqrt)
RandomForest 109.0293s   0.2501s     0.0263  (max_features=15)
ExtraTrees    53.3487s   0.4981s     0.0381  (max_features=sqrt)
ExtraTrees    93.1047s   0.3927s     0.0277  (max_features=15)
CART          16.5673s   0.0155s     0.0425  
```

```
master   train-time test-time error-rate
--------------------------------------------
RandomForest  73.9088s   0.3621s     0.0309  (max_features=sqrt)
ExtraTrees   103.1957s   0.3149s     0.0212  (max_features=sqrt)
CART          17.6760s   0.0194s     0.0426  
```

There is a decrease of performance due to the change in semantics. Increasing max_features allows to get back to previous error-rate.