Unify volumes and datasets

Volumes and datasets are two different concepts in float ("LVs" and "backups") but they both claim ownership to a directory associated with a service. We should at this point save the effort of creating this directory via Ansible, and introduce some different organization of service-associated concepts that suits us better.

One obstacle is that datasets can be something else other than paths, and it's nice to have a single view of the backup-able data. Perhaps we can reduce volumes to be simply LVs (no associated ownership, just a size), and make the "path"-type dataset the primary owner of the directory, with associated ownership and permissions (which we need in any case for restores).