Use just md<index> as the default volume name if a volume name is not defined.
The original change (https://review.opendev.org/c/openstack/ironic-python-agent/+/853182)
introduced an error:
mdadm: Value "/dev/md0" cannot be set as name. Reason: Not POSIX compatible.\n
This change fixes it.
Closes-Bug: #2073406
Change-Id: Ic8bd473801fcb92fc814f6ad4e1d6dc316783bf3