lib/lvm: don't use $VOLUME_GROUP variable anymore

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
diff --git a/doc/source/configuration.rst b/doc/source/configuration.rst
index 22809eb..391e5be 100644
--- a/doc/source/configuration.rst
+++ b/doc/source/configuration.rst
@@ -703,13 +703,13 @@
 ~~~~~~
 
 The logical volume group used to hold the Cinder-managed volumes is
-set by ``VOLUME_GROUP``, the logical volume name prefix is set with
+set by ``VOLUME_GROUP_NAME``, the logical volume name prefix is set with
 ``VOLUME_NAME_PREFIX`` and the size of the volume backing file is set
 with ``VOLUME_BACKING_FILE_SIZE``.
 
     ::
 
-        VOLUME_GROUP="stack-volumes"
+        VOLUME_GROUP_NAME="stack-volumes"
         VOLUME_NAME_PREFIX="volume-"
         VOLUME_BACKING_FILE_SIZE=10250M