instance-enb and instance-ue currently setup that slap_configuration with
LTE/NR defaults to be accessible from enb.jinja2.cfg and ue.jinja2.cfg. But we
will soon need to have access to those defaults from slaplte.jinja2 as well,
and it will break if left as is because when slaplte is imported from e.g.
enb.jinja2 - it will work, but when slaplte is imported from instance-enb it
will break because parent of instance-enb (instance.cfg) does not setup
defaults in slap_configuration at that level.

The fix is to either duplicate slap_configuration at instance.cfg level, or to
switch access to the defaults to go via original default_* parameters.

We go the second way for simplicity.