ps: add partial psutil 2.0 compatibility

The ps execution module uses the psutil Python module. It's API changed in the 2.0 version, but in the ps module the old API was used. This pull request gives the ps module some support both the 2.x and the 1.x version. It's just partial, but the test cases are passsing. Maybe the ps module will need some refactoring in the future.