Added hardware model detection for configurations

I provisioned a MacBook Air and Mac Pro short after each other with the same kitchenplan config. Since the hostname got set by attributes they got the same hostname. 

I modified the code that it will check in the system folder for a system specific YAML-file, like `config/system/MacPro3,1.yml` to allow different attributes per hardware-model.

My darwin shell knowledge is limited so I guess the command to retrieve the hardware model could be improved (see line 34).