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We are going to generate more and more type of files via slapos-render-config:
currently we generate only enb.cfg and gnb.cfg, but soon it will be also
ue-*.cfg, drb_*.cfg, asn etc.

So it will be handy to easily separate generated files from their source, to
make snapshots of generated stuff for comparison, and to also e.g. to be able
to copy generated configs to a callbox/simbox for testing.

-> Move the place where generated files are emitted from config/ to config/out/

/cc @lu.xu, @tomo, @xavier_thompson, @Daetalus
/reviewed-by @jhuge
/reviewed-on nexedi/slapos!1513