Fixed for..in loop in Linux collector

Yeah, RTFM... In Python, we must specify on which item we want to iterate in a directory (key, values or key/values)
http://stackoverflow.com/questions/3294889/iterating-over-dictionaries-for-loops-in-python/3294899#3294899
Merge pull request #102 from CentralReport/fix-disk-collector

Fixed for..in loop in the Linux collector