- fixing an issue introduced with CUDA-TK 7.0, when automatic selection
  of free gpu by os / library no longer works. Library always selects '0',
  which leads to crash in case of 'exclusive mode'.
- this fix is inspired by Dan Povey's fix for Kaldi:
  https://github.com/kaldi-asr/kaldi/commit/6548565445167e00125848f91d7da5f3f949b2a2
- it does a loop over gpus until a free gpu is taken.