Update metric names for Zuul / Gerrit graphite graphs

* The metric name hierarchy changed, with individual apps now
  sitting at the top level (stats_counts.gerrit -> gerrit).

* The input of the data now wraps most data points in an extra
  level (gerrit.patchset-created -> gerrit.patchset-created.count).

Change-Id: I5f7f13de4903c50bef72bfe0c02f8e040846392b