This adds a new variable that complements FDO_UPSTREAM_REPO. It will
default to whatever FDO_UPSTREAM_REPO is set to but turned into lower
case, as that is usually where the container image registry is located
it for a project.

It is still possible to set the FDO_UPSTREAM_REGISTRY when using the
templates, to override the derived default.

Closes: #22



Signed-off-by: Jonas Ådahl <jadahl@gmail.com>