Change "admin" literal for admin role to CONF admin_role

Currently, the Patrole framework always assumes that the admin
role is "admin". But this might not necessarily be the case.
The word "admin" is just a convention, but is nonetheless an
arbitrary designation for administration privileges.

Instead, the Patrole framework should take advantage of the
already-existing Tempest configuration option:

    cfg.StrOpt('admin_role',
               default='admin',
               help="Role required to administrate keystone."),

This patch changes instances of 'admin' (for identifying the
admin role) with ``CONF.identity.admin_role``. This patch doesn't
make changes to 'admin' in unit tests, as that's not necessary,
but instead uses ``CONF.set_override`` to change the ``admin_role``
to "admin".

Closes-Bug: #1680294
Change-Id: Ia4431c2a16892a60fe10bb7e8495e7e384e552c1
diff --git a/patrole_tempest_plugin/rbac_utils.py b/patrole_tempest_plugin/rbac_utils.py
index 55a5599..4cddb8d 100644
--- a/patrole_tempest_plugin/rbac_utils.py
+++ b/patrole_tempest_plugin/rbac_utils.py
@@ -160,7 +160,7 @@
         for role in available_roles['roles']:
             if role['name'] == CONF.rbac.rbac_test_role:
                 rbac_role_id = role['id']
-            if role['name'] == 'admin':
+            if role['name'] == CONF.identity.admin_role:
                 admin_role_id = role['id']
 
         if not admin_role_id or not rbac_role_id: