ceph-volume: honour osd_dmcrypt_key_size option

ceph-volume: honour osd_dmcrypt_key_size option

ceph-volume doesn't honour osd_dmcrypt_key_size.
It means the default size is always applied.

It also changes the default value in `get_key_size_from_conf()`

From cryptsetup manpage:

> For XTS mode you can optionally set a key size of 512 bits with the -s option.

Using more than 512bits will end up with the following error message:

```
Key size in XTS mode must be 256 or 512 bits.
```

Fixes: https://tracker.ceph.com/issues/54006
Signed-off-by: Guillaume Abrioux <gabrioux@redhat.com>
(cherry picked from commit 47c33179f9a15ae95cc1579a421be89378602656)