undercloud: no default for registry-mirror param
undercloud: no default for registry-mirror param

Registry-mirror does need to not have any default value.

In internal/custom builds value is expected to be obtained
from the compose/puddle's container-image-prepare yamls.
Otherwise TripleO defaults are to be honored for public content.

Since this default values is "user parameter" it takes highest priority
inside code, making any detection of per-compose or use of product
defaults impossible, always overriding them.

Change-Id: I7c66fd674ba9ae8e8ee2b2e2bec1e931358fdf0e

redhat-openstack/infrared,redhat-openstack/infrared,redhat-openstack/infrared