Make StatsD module backend-agnostic; make NavigationTiming instance Ganglia-only

The StatsD module currently makes the Graphite back-end nonoptional. This makes
it impossible to use the module to provision a Ganglia-only StatsD instance.
This patch makes leaves the choice of backends up to the instantiator of the
class and updates the configuration of the hafnium StatsD instance to be
Ganglia-only.

Change-Id: Ie5b11292bd6294506d7afb1ed2fb59feb6f2bc3f