CentOS configuration updates & fetch.py feature

Minor update to #! in plsync.py to use python from PATH, rather than /usr/bin/python.
Updates to slices.py and sites.py in plsync for CentOS configuration.  Added additional notes. (I know the wide columns are not exciting, but in defense, these are config files not code, and meant to be viewed 'all at once' easily.)
Add option to fetch.py --nodelist "host1,host2,host3" to accept an explicit list of host names on the command line.