We should be using $VOLUME_GROUP_NAME instead since Icehouse.
$VOLUME_GROUP_NAME has been introduced in
I93b8ef32832269d730c76a6dc24ddb4f20c6d9df and $VOLUME_GROUP is nowadays
only use as a fallback to $VOLUME_GROUP_NAME.
As a code comment in lib/lvm says it we kept the $VOLUME_GROUP around as
"for compatibility with icehouse-generation Grenade". Icehouse is long
gone so now seems a good time to remove any usage of $VOLUME_GROUP.
Change-Id: Id3051b5a196c45266c39fde4f08401aaacf0f6bd