default host, fixes attribute error

I was getting this python error, because i don't have a config file.

  File "/home/brenda/projects/plow/lib/python/plow/client/conf.py", line 22, in _init
    host_list = [h.strip() for h in get('plow', 'hosts').strip(',').split(',')]
AttributeError: 'NoneType' object has no attribute 'strip'


So i gave hosts a default value, and now a more informative error occurs.
AssertionError: Configuration is missing a 'plow' section

This gives the next person more clues to work with.