Fix config group for cinder multi_backend

This commit just updates the config group for the multi_backend option.
Tempest change d5c9602b created a volume-feature-enabled group and moved this
volume there but devstack was never updated with the change.

Closes-Bug: #1276326

Change-Id: Icf2e96783feec4edbd4d477f8492651cd9bb3f01
diff --git a/lib/tempest b/lib/tempest
index 06183b1..6fa35d1 100644
--- a/lib/tempest
+++ b/lib/tempest
@@ -323,7 +323,7 @@
     fi
     CINDER_MULTI_LVM_BACKEND=$(trueorfalse False $CINDER_MULTI_LVM_BACKEND)
     if [ $CINDER_MULTI_LVM_BACKEND == "True" ]; then
-        iniset $TEMPEST_CONFIG volume multi_backend_enabled "True"
+        iniset $TEMPEST_CONFIG volume-feature-enabled multi_backend "True"
         iniset $TEMPEST_CONFIG volume backend1_name "LVM_iSCSI"
         iniset $TEMPEST_CONFIG volume backend2_name "LVM_iSCSI_2"
     fi