Adds support for setting the system scope to user role assignments.
Also updates the domain assignment so it can be customised.

Note that the scope assignments follow the precedence of
project->domain->system [1]. As such, the previous default value of
domain was being ignored as we always set a project, so the removal of
the default domain in this patch has no effect on existing behaviour.

1. https://docs.ansible.com/ansible/latest/collections/openstack/cloud/role_assignment_module.html#parameter-system

Change-Id: Ie7fe78ab67b1bf8a19def25fef321de5c2d80aa9