Fix: don't hardcode admin role name, use config option instead

Change-Id: I7d80de4f8d1df97901041aaccadae2a426c868eb
diff --git a/tempest/api/identity/admin/v3/test_default_project_id.py b/tempest/api/identity/admin/v3/test_default_project_id.py
index f74b30d..62a2a9c 100644
--- a/tempest/api/identity/admin/v3/test_default_project_id.py
+++ b/tempest/api/identity/admin/v3/test_default_project_id.py
@@ -65,7 +65,7 @@
                          "doesn't have domain id " + dom_id)
 
         # get roles and find the admin role
-        admin_role = self.get_role_by_name("admin")
+        admin_role = self.get_role_by_name(CONF.identity.admin_role)
         admin_role_id = admin_role['id']
 
         # grant the admin role to the user on his project