Enables the multibackend Cinder tests in tempest when useful.

The change adds in lib/tempest an optional dependency on the var
$CINDER_MULTI_LVM_BACKEND which, if set, enables the multibackend
tests in the tempest config.

Change-Id: Iccafd00f5adabbbc2309fa72664bf29440744d91
diff --git a/lib/cinder b/lib/cinder
index 7e9c2ba..6636397 100644
--- a/lib/cinder
+++ b/lib/cinder
@@ -194,7 +194,7 @@
         iniset $CINDER_CONF lvmdriver-1 volume_backend_name LVM_iSCSI
         iniset $CINDER_CONF lvmdriver-2 volume_group $VOLUME_GROUP2
         iniset $CINDER_CONF lvmdriver-2 volume_driver cinder.volume.drivers.lvm.LVMISCSIDriver
-        iniset $CINDER_CONF lvmdriver-2 volume_backend_name LVM_iSCSI
+        iniset $CINDER_CONF lvmdriver-2 volume_backend_name LVM_iSCSI_2
     else
         iniset $CINDER_CONF DEFAULT volume_group $VOLUME_GROUP
         iniset $CINDER_CONF DEFAULT volume_name_template ${VOLUME_NAME_PREFIX}%s