Assign a volume to each purpose and disable thin provisioning in LVM

[cmd-ntrf]

Following feedback by @verdurin in #71, I probably went too far in simplification by pooling all volumes in a single group and activating thin provisioning to simplify volumes sizing.

We need a compromise between earlier mkfs in cloud-init solution and LVM. The device id of the volumes could be configured in the hieradata instead of the size. The device ids for each purpose (home, project or scratch) should be a list, so more than one volume could be assigned to each pool eventually.