curtin fails to recreate existing logical volumes

[frukto]

curtin fails to recreate existing logical volumes. If a logical volume already exist and is configured with "preserve: false" it should be deleted and then (re-)created.

Now curtin tries to create the volume and fails because a logical volume with this name already exists.

See "lvm_partition_handler" in https://github.com/canonical/curtin/blob/master/curtin/commands/block_meta.py

Use case, keep only some logical volumes, but others not:

    - type: lvm_partition
      id: lv_01
      name: lv_01
      volgroup: vg_01
      size: 1024M
      preserve: true

    - type: lvm_partition
      id: lv_02
      name: lv_02
      volgroup: vg_01
      size: 1024M
      preserve: false

All "underlying" objects (disks, partitions, volume groups) need "preserve: true" and "wipe: false" (i.e unset). So all present logical volumes will be visible to curtin.