Disable tempest backup tests if c-bak unavailable

This will update the tempest config to not run the cinder backup
tests when the c-bak service is not enabled.

Change-Id: I0b6486f1222afa7ae9bd9d13c7d3648d2b870710
diff --git a/lib/tempest b/lib/tempest
index c8eebfc..596750b 100644
--- a/lib/tempest
+++ b/lib/tempest
@@ -314,8 +314,8 @@
     iniset $TEMPEST_CONFIG scenario large_ops_number ${TEMPEST_LARGE_OPS_NUMBER:-0}
 
     # Volume
-    if is_service_enabled c-bak; then
-        iniset $TEMPEST_CONFIG volume volume_backup_enabled "True"
+    if ! is_service_enabled c-bak; then
+        iniset $TEMPEST_CONFIG volume-feature-enabled backup False
     fi
     CINDER_MULTI_LVM_BACKEND=$(trueorfalse False $CINDER_MULTI_LVM_BACKEND)
     if [ $CINDER_MULTI_LVM_BACKEND == "True" ]; then