docs: Add tip about creating the creator role

The default example mapping includes the "creator" role, but it does not
exist by default after completing most of the OpenStack setup. To avoid
errors when logging in with a valid Rackspace username/password, manually
create the "creator" role. Without it, Keystone will return the following
error:

ERROR keystone.auth.plugins.mapped [None-4e390453-680f-4a5d-a315-2a0ac7693033
- - - - - -] Role creator was specified in the mapping but does not exist.
All roles specified in a mapping must exist before assignment.

Author: LukeRepko

docs/openstack-keystone-federation.md

@@ -24,6 +24,16 @@ You're also welcome to generate your own mapping to suit your needs; however, if
 
 The example mapping **JSON** file can be found within the genestack repository at `/opt/genestack/etc/keystone/mapping.json`.
 
+!!! tip "Creating the `creator` role"
+
+    The creator role does not exist by default, but is included in the example
+    mapping. One must create the creator role in order to prevent authentication
+    errors if using the mapping "as is".
+
+    ``` shell
+    openstack --os-cloud default role create creator
+    ```
+
 ## Now register the mapping within Keystone
 
 ``` shell