Track event counts in Graphite

This patch defines a new role class, 'role::eventlogging::graphite', and
assigns it to hafnium. By declaring the class a node becomes an
EventLogging git-deploy deployment target (via
modules/eventlogging/manifests/init.pp). It also provisions a single
EventLogging event consumer which will maintain a count of incoming
events, broken down by schema, in Graphite, via StatsD.

Change-Id: I558d39e0453c44846d223994a74d8921ef9f0585