ceph-volume: fix tags dict output in `lvm list`

ceph-volume: fix tags dict output in `lvm list`

Default value for `--crush-device-class` is `None`.
When not passing this parameter, ceph-volume sets the
value "None" in the lv tags.
Therefore, ceph-volume will output that value with calling
`ceph-volume lvm list --format json`

For instance:
```
    "1": [
        {
            "devices": [
                "/dev/sdc"
            ],
            "lv_name": "osd-data-5a4a34f5-5733-4c69-b439-edb48e31a45f",
            "lv_path": "/dev/ceph-aeb16fc3-9ac2-4126-ab66-bf920d101ea4/osd-data-5a4a34f5-5733-4c69-b439-edb48e31a45f",
            "lv_size": "49.00g",
            "lv_tags": "ceph.block_device=/dev/ceph-aeb16fc3-9ac2-4126-ab66-bf920d101ea4/osd-data-5a4a34f5-5733-4c69-b439-edb48e31a45f,ceph.block_uuid=E9hZNU-80Zz-PiER-iWN3-jSIU-krEN-khwU3x,ceph.cephx_lockbox_secret=,ceph.cluster_fsid=40fe4af5-0408-444b-843c-0926d550d1f1,ceph.cluster_name=ceph,ceph.crush_device_class=None,ceph.encrypted=0,ceph.osd_fsid=39680838-19df-4e50-9bb6-46b093d5b52b,ceph.osd_id=1,ceph.type=block,ceph.vdo=0",
            "lv_uuid": "E9hZNU-80Zz-PiER-iWN3-jSIU-krEN-khwU3x",
            "name": "osd-data-5a4a34f5-5733-4c69-b439-edb48e31a45f",
            "path": "/dev/ceph-aeb16fc3-9ac2-4126-ab66-bf920d101ea4/osd-data-5a4a34f5-5733-4c69-b439-edb48e31a45f",
            "tags": {
                "ceph.block_device": "/dev/ceph-aeb16fc3-9ac2-4126-ab66-bf920d101ea4/osd-data-5a4a34f5-5733-4c69-b439-edb48e31a45f",
                "ceph.block_uuid": "E9hZNU-80Zz-PiER-iWN3-jSIU-krEN-khwU3x",
                "ceph.cephx_lockbox_secret": "",
                "ceph.cluster_fsid": "40fe4af5-0408-444b-843c-0926d550d1f1",
                "ceph.cluster_name": "ceph",
                "ceph.crush_device_class": "None",
```

ceph-volume should print `"ceph.crush_device_class": "",` instead of `"ceph.crush_device_class": "None",`

Fixes: https://tracker.ceph.com/issues/53425
Signed-off-by: Guillaume Abrioux <gabrioux@redhat.com>