Add a debian-mirror heat template
Add a debian-mirror heat template

Populate defaults for the debian-mirror image-element using a heat
template.

Change-Id: I28de1d04be7292dad1bc448ff8bbde634cebd6f3

rdo-management/tripleo-heat-templates,rdo-management/tripleo-heat-templates,trozet/tripleo-heat-templates,redhat-openstack/tripleo-heat-templates,dprince/tripleo-heat-templates,bcrochet/tripleo-heat-templates,wyg3958/tripleo-heat-templates,bcrochet/tripleo-heat-templates,trozet/opnfv-tht,openstack/tripleo-heat-templates,trozet/opnfv-tht,wyg3958/tripleo-heat-templates,trozet/tripleo-heat-templates,trozet/opnfv-tht,redhat-openstack/tripleo-heat-templates,dprince/tripleo-heat-templates,bcrochet/tripleo-heat-templates,openstack/tripleo-heat-templates