Explicitly manage /srv/graphite/storage perms

This has stemmed out of the Vagrant setup i'm doing for the
monitoring system -- previously /srv/graphite/storage would be
automatically created with the right perms. Now we need to account
for it possibly being there before we start - with incorrect ownership.
Merge pull request #21 from ministryofjustice/manage_storage_dir

Explicitly manage /srv/graphite/storage perms