ceph_volume: fix multiple db/wal/journal devices

ceph_volume: fix multiple db/wal/journal devices

When using the lvm batch ceph-volume subcommand with dedicated devices
for filestore (journal) or bluestore (db/wal) then the list of devices
is convert to a string instead of being extended via an iterable.
This was working with only one dedicated device but starting with more
then the ceph_volume module fails.

TASK [ceph-osd : use ceph-volume lvm batch to create bluestore osds] **
fatal: [xxxxxx]: FAILED! => changed=true
  cmd:
  - ceph-volume
  - --cluster
  - ceph
  - lvm
  - batch
  - --bluestore
  - --yes
  - --prepare
  - --osds-per-device
  - '4'
  - /dev/nvme2n1
  - /dev/nvme3n1
  - /dev/nvme4n1
  - /dev/nvme5n1
  - /dev/nvme6n1
  - --db-devices
  - /dev/nvme0n1 /dev/nvme1n1
  - --report
  - --format=json
  msg: non-zero return code
  rc: 2
  stderr: |2-
     stderr: lsblk: /dev/nvme0n1 /dev/nvme1n1: not a block device
     stderr: error: /dev/nvme0n1 /dev/nvme1n1: No such file or directory
     stderr: Unknown device, --name=, --path=, or absolute path in /dev/ or /sys expected.
    usage: ceph-volume lvm batch [-h] [--db-devices [DB_DEVICES [DB_DEVICES ...]]]
                                 [--wal-devices [WAL_DEVICES [WAL_DEVICES ...]]]
                                 [--journal-devices [JOURNAL_DEVICES [JOURNAL_DEVICES ...]]]
                                 [--no-auto] [--bluestore] [--filestore]
                                 [--report] [--yes] [--format {json,pretty}]
                                 [--dmcrypt]
                                 [--crush-device-class CRUSH_DEVICE_CLASS]
                                 [--no-systemd]
                                 [--osds-per-device OSDS_PER_DEVICE]
                                 [--block-db-size BLOCK_DB_SIZE]
                                 [--block-wal-size BLOCK_WAL_SIZE]
                                 [--journal-size JOURNAL_SIZE] [--prepare]
                                 [--osd-ids [OSD_IDS [OSD_IDS ...]]]
                                 [DEVICES [DEVICES ...]]
    ceph-volume lvm batch: error: Unable to proceed with non-existing device: /dev/nvme0n1 /dev/nvme1n1

So the dedicated device list is considered as a single string.

This commit also adds the journal_devices, block_db_devices and
wal_devices documentation to the ceph_volume module.

Closes: https://bugzilla.redhat.com/show_bug.cgi?id=1816713
Signed-off-by: Dimitri Savineau <dsavinea@redhat.com>
(cherry picked from commit 760b6cd7b0b671d63e2d6ea32ec336edd2f8cd32)