Add note about image_name_prefix

Author: Alex-Welsh

etc/kayobe/kolla/globals.yml

@@ -1,6 +1,10 @@
 # yamllint disable-file
 ---
 
+# TODO(Alex-Welsh): Check whether this is necessary and this is the correct
+# place to put it
+#image_name_prefix: "{{ os_distribution }}"
+
 # To work around issue of trying to install docker from
 # empty pulp server, use upstream docker dnf repo on
 # non-overcloud hosts