Inconsistency for Num threads calculation if they don't specified

We have few places in code with code like:
```
_threads = Math.Max(1, args.Threads ?? (Environment.ProcessorCount / 2));
```
in some places it's ```Math.Max(2, Environment.ProcessorCount - 1)```

in others it's`Args.NumThreads ?? Environment.ProcessorCount;`

which is a bit strange. It's either half of cores, all cores, or we left one core to be free.
KMeans, SdcaRegression, LinearClassifier , and BinaryLoader use half of available cores which leads to 50% CPU utilization.
@TomFinley, @GalOshri , @glebuk