Add Grafana scripted dashboards; plus method to override default dashboard; graphite.data_dir

These should be useful for all standard graphite/collectd deployments:

* overview.js -- provide an overview of core metrics for nodes under a 'env' prefix
* instance.js -- detailed view of a specific node. Linked to via overview.js.
* monitoring_health.js -- key graphs showing the state of the monitoring system

I've updated the default route to point to overview.js, as the previous default was pretty useless.

Also includes a change to allow overriding of the graphite.data_dir -- the default remaining as /srv/graphite/storage