Collectd set prefix

* Allow setting pillar values to change the index name grafana reads from in the pillar
* Allow setting the collectd prefix in the pillar i.e. so multiple projects can coexist on one monitoring server.
Paul added it here:
https://github.com/ministryofjustice/metrics-formula/commit/e574fd61f683bb3dec3525af762c6e743c7eb363

The default is an _