dynamic host volumes: CE side of quota tweaks (#24972)

* quota spec:
  if `region_limit.storage.host_volumes` is set,
  do not require that `variables` also be set,
  and vice versa.
* subtract from quota usage on volume delete
* stub CE quota subtraction method

command/quota_apply.go

@@ -355,6 +355,8 @@ func parseQuotaMegabytes(raw any) (int, error) {
 		return int(b >> 20), nil
 	case int:
 		return val, nil
+	case nil:
+		return 0, nil
 	default:
 		return 0, fmt.Errorf("invalid type %T", raw)
 	}

nomad/state/state_store_host_volumes.go

@@ -142,6 +142,11 @@ func (s *StateStore) DeleteHostVolume(index uint64, ns string, id string) error
 			}
 		}
 
+		err = s.subtractVolumeFromQuotaUsageTxn(txn, index, vol)
+		if err != nil {
+			return err
+		}
+
 		err = txn.Delete(TableHostVolumes, vol)
 		if err != nil {
 			return fmt.Errorf("host volume delete: %w", err)

nomad/state/state_store_host_volumes_ce.go

@@ -14,3 +14,7 @@ func (s *StateStore) EnforceHostVolumeQuota(_ *structs.HostVolume, _ *structs.Ho
 func (s *StateStore) enforceHostVolumeQuotaTxn(_ Txn, _ uint64, _ *structs.HostVolume, _ *structs.HostVolume, _ bool) error {
 	return nil
 }
+
+func (s *StateStore) subtractVolumeFromQuotaUsageTxn(_ Txn, _ uint64, _ *structs.HostVolume) error {
+	return nil
+}