Use service_domain in [service_user] section (#813)

The keystone charm currently creates two service users, one for the
service domain (for v3 authentication), and the other for the default
domain (for v2 authentication).

This patch updates the [service_user] config to use the service
domain.

Closes-Bug: #2026202

charmhelpers/contrib/openstack/templates/section-service-user

@@ -3,8 +3,8 @@
 send_service_user_token = true
 auth_type = password
 auth_url = {{ auth_protocol }}://{{ auth_host }}:{{ auth_port }}
-project_domain_id = default
-user_domain_id = default
+project_domain_name = service_domain
+user_domain_name = service_domain
 project_name = {{ admin_tenant_name }}
 username = {{ admin_user }}
 password = {{ admin_password }}