labmon: Store graphite data in /srv/carbon

- Sets up a symlink from /var/lib/carbon to pseudo-mimic how
  things are set up in tungsten
- Introduces a parameter to role::graphite that lets us set the
  storage_dir

Change-Id: I8fa328868bd73d51dd5e3291ff347d33be57b74a
Merge "labmon: Store graphite data in /srv/carbon" into production