Tweak ParamikoMachine for python3 compatibility

I am new to Python, so I'm not entirely familiar with the reasons why you need a byte string here, since (under python 3 at least) stdout and stderr contain utf strings. But I'll go along with it and convert them to byte strings too, which ought to also work in Python 2.

Note that I am using scottkmaxwell/paramiko for python 3 compatibility: https://github.com/scottkmaxwell/paramiko/tree/py3-support-without-py25