tempest: set admin_system = all (#200)

The system scope on which an admin user has an admin role
assignment, if any. Valid values are 'all' or None. This must be set
to 'all' if using the [oslo_policy]/enforce_scope=true option for
the identity service. (string value)

Signed-off-by: Christian Berendt <berendt@osism.tech>

roles/tempest/templates/tempest.conf.j2

@@ -16,6 +16,7 @@ admin_username = {{ tempest_admin_username }}
 admin_password = {{ tempest_admin_password }}
 admin_project_name = {{ tempest_admin_project_name }}
 admin_domain_name = {{ tempest_admin_domain_name }}
+admin_system = all
 
 {% if tempest_enable_nova | bool -%}
 [compute]