Allow specification of graphite.data_dir

Previously we were hardcoded to use /srv/graphite/storage, which can
wind up filling the root fs.

Allow relocation to whereever, but this ideally should be /data/graphite
for consistency.

NB: this does not handle migration of your data from the old to the new location.
    See UPGRADING.md for instructions on how to do this.
Add grafana scripted dashboards

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
Allow override of grafana default route.

Make the default 'overview.js', so it's actually useful.
Merge branch 'master' into grafana_scripts
Merge branch 'master' into grafana_scripts

Conflicts:
	metrics/map.jinja
Remove exec permission from dashboard scripts

There is no reason to have +x on the grafana dashboard JS files, so
remove it.
Merge branch 'master' into grafana_scripts

Conflicts:
	CHANGELOG.md
Allow selection of specific nodes for overview

Useful for dashboard screens, limit the nodes returned in the dashboard in
a bookmarkable manner.
Merge branch 'master' into grafana_scripts

Conflicts:
	CHANGELOG.md
Merge pull request #31 from ministryofjustice/grafana_scripts

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