Plumbing for setting own docker registry

* as an operator I can specify an IP to use for docker registry
* as a multi-hypervisor openstack I want to refer back to central
docker registry

Also remove auto-discovery on docker-registry container

Co-Authored-By: Julien Vey <julien.vey@numergy.com>

Change-Id: Iba0e00bda1742aa428b4064bfdb1f80ce1c374c9
Merge "Plumbing for setting own docker registry"