For its operation slapos-render-config mimics jinja2 templates rendering
done for real in slapos. But to do so it currently duplicates the code
from slapos.recipe.template.

We can do the same by reusing slapos.recipe.template instead of
duplicating code from there.

-> Do it.

The output of generated gnb.cfg is the same before and after this patch.

/cc @xavier_thompson, @Daetalus
/reviewed-by @jhuge, @lu.xu, @tomo
/reviewed-on nexedi/slapos!1462