Use service_domain in [service_user] section

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
(cherry picked from commit 3ed0675)

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 }}