nixos/graphite: carbon, use syslog for logging

Twisted provides option to log with syslog, this enables nicer logging.
Imagine what happens in a case of exception. If logs are written to stdout,
traceback won't be merged thus giving ugly logs. This commit fixes that.
This is also one of the official ways of starting carbon, so no worries.
Merge pull request #1073 from offlinehacker/nixos/graphite/fix_carbon

nixos/graphite: carbon, use syslog for logging