Update service client access test_dns_integration

This commit updates the Designate service client access to use the
tempest registry instead of clients added to the client manager.

Depends-On: https://review.opendev.org/c/openstack/designate-tempest-plugin/+/800280
Change-Id: Ie72e31e8e9081d305a5fb79ae6296546e29a0ee8
1 file changed