Fix SPACK-27 & remove dependence on check_output
Fix SPACK-27 & remove dependence on check_output

- subprocess.check_output is python 2.7 only
- Spack checks for existence of requested prefix, creates it if it does not exist.

LLNL/spack,tmerrick1/spack,mfherbst/spack,EmreAtes/spack,mfherbst/spack,tmerrick1/spack,matthiasdiener/spack,skosukhin/spack,krafczyk/spack,TheTimmy/spack,EmreAtes/spack,matthiasdiener/spack,LLNL/spack,mfherbst/spack,tmerrick1/spack,lgarren/spack,krafczyk/spack,lgarren/spack,lgarren/spack,skosukhin/spack,EmreAtes/spack,EmreAtes/spack,TheTimmy/spack,matthiasdiener/spack,TheTimmy/spack,TheTimmy/spack,lgarren/spack,skosukhin/spack,mfherbst/spack,krafczyk/spack,tmerrick1/spack,iulian787/spack,iulian787/spack,matthiasdiener/spack,krafczyk/spack,krafczyk/spack,LLNL/spack,mfherbst/spack,skosukhin/spack,lgarren/spack,EmreAtes/spack,LLNL/spack,iulian787/spack,matthiasdiener/spack,TheTimmy/spack,skosukhin/spack,iulian787/spack,tmerrick1/spack,iulian787/spack,LLNL/spack