Add note about ironic config options

Add a note to config.py: ironic config options
need to be kept here (in tempest) until Liberty EOL.

Change-Id: Iac0da6ce6ffb14b69b7cedac2356d6d4cc6b6df8
diff --git a/tempest/config.py b/tempest/config.py
index 6942172..14a6ad2 100644
--- a/tempest/config.py
+++ b/tempest/config.py
@@ -1189,6 +1189,11 @@
                                     'live_migration, pause, rescue, resize '
                                     'shelve, snapshot, and suspend')
 
+
+# NOTE(deva): Ironic tests have been ported to tempest-lib. New config options
+#             should be added to ironic/ironic_tempest_plugin/config.py.
+#             However, these options need to remain here for testing stable
+#             branches until Liberty release reaches EOL.
 BaremetalGroup = [
     cfg.StrOpt('catalog_type',
                default='baremetal',