ceph-volume: fix argparse dmcrypt opts: use str type

argparse's `type` has to be a callable that turns the CLI string into the
stored value.

Optional[str] is a typing construct. Newer argparse treats that specially
and fails with "invalid Optional value".

default=None already means the flag is optional.

Fixes: https://tracker.ceph.com/issues/76433

Signed-off-by: Guillaume Abrioux <gabrioux@ibm.com>