Add system scope for admin auth

Keystone is moving away from using either project-scope or domain-scope
for the main cloud administrator user, and instead moving toward the
admin user having a role assignment on the "system" scope[1]. This will
mean that no particular project or domain is special, and instead the
cloud administrator scopes to the system in order to make
deployment-wide changes. Keystone has now migrated all of its policies
to understand system scope[2], and if a deployment sets
[oslo_policy]/enforce_scope=true in keystone.conf and uses the new
policies, an admin user scoped to the admin project will not be able to
create dynamic credentials for tempest.

This patch adds a new parameter ``[auth]/admin_system`` to indicate that
neither the ``admin_project`` or ``admin_domain`` parameters apply to
the admin user and that the user should instead authenticate with the
system scope. This also adds ``admin_user_domain_name`` so that the
admin user can be found in its domain (namespace) without setting
``domain_name``, and for completeness also adds
``admin_project_domain_name`` so that ``domain_name`` could be omitted
even if using project scope.

[1] http://specs.openstack.org/openstack/keystone-specs/specs/keystone/queens/system-scope.html
[2] https://bugs.launchpad.net/keystone/+bugs?field.status%3Alist=FIXRELEASED&field.tag=system-scope

Depends-on: https://review.opendev.org/739262

Change-Id: I840b273c37ca7cc4592c43813abfb424337e2836
9 files changed