ceph-volume: support splitting db even on collocated scenario

ceph-volume: support splitting db even on collocated scenario

This change enables ceph-volume to create OSDs where the DB is
explicitly placed on a separate LVM partition, even in collocated
scenarios (i.e., block and DB on the same device).
This helps mitigate BlueStore fragmentation issues.

Given that ceph-volume can't automatically predict a proper default size for the db device,
the idea is to use the `--block-db-size` parameter:

Passing `--block-db-size` and `--db-devices` makes ceph-volume create db devices
on dedicated devices (current implementation):

```
Total OSDs: 2

  Type            Path                                                    LV Size         % of device
----------------------------------------------------------------------------------------------------
  data            /dev/vdb                                                200.00 GB       100.00%
  block_db        /dev/vdd                                                4.00 GB         2.00%
----------------------------------------------------------------------------------------------------
  data            /dev/vdc                                                200.00 GB       100.00%
  block_db        /dev/vdd                                                4.00 GB         2.00%
```

Passing `--block-db-size` without `--db-devices` makes ceph-volume create a separate
LV for db device on the same device (new behavior):
```
Total OSDs: 2

  Type            Path                                                    LV Size         % of device
----------------------------------------------------------------------------------------------------
  data            /dev/vdb                                                196.00 GB       98.00%
  block_db        /dev/vdb                                                4.00 GB         2.00%
----------------------------------------------------------------------------------------------------
  data            /dev/vdc                                                196.00 GB       98.00%
  block_db        /dev/vdc                                                4.00 GB         2.00%
```

This new behavior is supported with the `--osds-per-device` parameter:

```
Total OSDs: 4

  Type            Path                                                    LV Size         % of device
----------------------------------------------------------------------------------------------------
  data            /dev/vdb                                                96.00 GB        48.00%
  block_db        /dev/vdb                                                4.00 GB         2.00%
----------------------------------------------------------------------------------------------------
  data            /dev/vdb                                                96.00 GB        48.00%
  block_db        /dev/vdb                                                4.00 GB         2.00%
----------------------------------------------------------------------------------------------------
  data            /dev/vdc                                                96.00 GB        48.00%
  block_db        /dev/vdc                                                4.00 GB         2.00%
----------------------------------------------------------------------------------------------------
  data            /dev/vdc                                                96.00 GB        48.00%
  block_db        /dev/vdc                                                4.00 GB         2.00%
```

Fixes: https://tracker.ceph.com/issues/69996
Signed-off-by: Guillaume Abrioux <gabrioux@ibm.com>