Add DockerRegistryMirror parameter
Add DockerRegistryMirror parameter

We want to use this for the containerized undercloud to
be able to consume an in-rack registry mirror for
our CI jobs.

Change-Id: Ia0a2b4a2ddd99c9ee9b71875b144824aa7543da1

openstack/tripleo-heat-templates,openstack/tripleo-heat-templates,dprince/tripleo-heat-templates,dprince/tripleo-heat-templates