Add variable override for OPENSTACK_KEYSTONE_ADMIN_ROLES

As Horizon treats admins differently and can't discern what roles are
administrative in nature, we have to tell it if there are roles
beyond/besides "admin".

If not overriden by the user, then the default is `['admin']` as seen
in the code:
https://github.com/openstack/horizon/blob/stable/mitaka/openstack_dashboard/utils/identity.py#L20-L25.

Closes-Bug: #1614213

Change-Id: I5e475db52be7d6390a1ab29a08b58fc102e16037
Co-Author: Corey Wright <corey.wright@rackspace.com>

defaults/main.yml

@@ -252,3 +252,6 @@ horizon_role_project_group: horizon_all
 
 # Set arbitrary horizon configuration options
 horizon_config_overrides: {}
+
+horizon_keystone_admin_roles:
+  - admin

releasenotes/notes/add-keystone-admin-roles-setting-83198a721c64ee3c.yaml

@@ -0,0 +1,5 @@
+---
+features:
+  - The ``horizon_keystone_admin_roles`` variable is added to support the
+    ``OPENSTACK_KEYSTONE_ADMIN_ROLES`` list in the horizon_local_settings.py
+    file.

templates/horizon_local_settings.py.j2

@@ -738,3 +738,9 @@ DEFAULT_THEME = '{{ horizon_default_theme }}'
 
 # Set arbitrary horizon configuration options
 HORIZON_CONFIG.update({{ horizon_config_overrides }})
+
+OPENSTACK_KEYSTONE_ADMIN_ROLES = [
+{% for item in horizon_keystone_admin_roles %}
+  '{{ item }}',
+{% endfor %}
+]