Expose role tags via heat RoleTags output

This will enable easier consumption of the tags e.g via
the dynamic ansible inventory or workflows that run ansible

Change-Id: I8810240d012ff369991e5e6098e36aa3713e2615

overcloud.j2.yaml

@@ -960,6 +960,12 @@ outputs:
 {% for role in roles %}
       {{role.name}}: {get_attr: [{{role.name}}NetworkHostnameMap, value]}
 {% endfor %}
+  RoleTags:
+    description: Tags for each role, as defined in roles_data.yaml
+    value:
+{%- for role in roles %}
+      {{role.name}}: {{role.tags|default([])}}
+{%- endfor %}
   ServerOsCollectConfigData:
     description: The os-collect-config configuration associated with each server resource
     value: {get_attr: [ServerOsCollectConfigData, value]}