report stats from shell scripts to statsd, not carbon

* Scripts writing to Carbon need to be aware of how Carbon is aggregating the
  particular metric and whether anything else is liable to be reporting the
  metric other than the currently executing shell script. StatsD manages state
  for you, so it's simpler and harder to screw up.
* Rename MW_CARBON_{HOST,PORT} to MW_STATSD_{HOST,PORT}

Change-Id: I933a31e5982f742d5bf41b914decb3b88477bce7