enable volume backup tests if c-bak is enabled

this will set to True the tempest volume/volume_backup_enabled
option if c-bak is in ENABLED_SERVICES

Change-Id: I69931d668411fc8144d0fdb2f58ad9b6e987f793
diff --git a/lib/tempest b/lib/tempest
index 50289b6..e48ccf2 100644
--- a/lib/tempest
+++ b/lib/tempest
@@ -286,6 +286,9 @@
     iniset $TEMPEST_CONF scenario large_ops_number ${TEMPEST_LARGE_OPS_NUMBER:-0}
 
     # Volume
+    if is_service_enabled c-bak; then
+        iniset $TEMPEST_CONF volume volume_backup_enabled "True"
+    fi
     CINDER_MULTI_LVM_BACKEND=$(trueorfalse False $CINDER_MULTI_LVM_BACKEND)
     if [ $CINDER_MULTI_LVM_BACKEND == "True" ]; then
         iniset $TEMPEST_CONF volume multi_backend_enabled "True"