clean.sh: use $VOLUME_GROUP_NAME instead of $VOLUME_GROUP
clean.sh: use $VOLUME_GROUP_NAME instead of $VOLUME_GROUP

We should be using $VOLUME_GROUP_NAME instead since Icehouse.
$VOLUME_GROUP_NAME has been introduced in devstack in
I93b8ef32832269d730c76a6dc24ddb4f20c6d9df and $VOLUME_GROUP is nowadays
only use as a fallback to $VOLUME_GROUP_NAME.

This is related to this devstack change
Id3051b5a196c45266c39fde4f08401aaacf0f6bd which tries to remove all
occurences of $VOLUME_GROUP in devstack.

Change-Id: I438973d559f7569ea5602091ed837e3d3959bf7b

openstack-dev/grenade,openstack-dev/grenade