[core.config] Remove type checking of `arg' in `config.app()'.
(It's done in `parse_app_arg'.)
[lib.json] Added back encoding support.
[JSON config] Replace all uses of `dofile' and `loadstring'
with the equivalent applications of `lib.json.decode'.

Provide central `lib.load_json' and `lib.store_json' to read
and write JSON files and use `store_json' to generate zone
configurations in `designs/neutron/neutron2snabb'.

Replace leftover string app conf arguments and convert test
fixtures to JSON. While at it I extended the `neutron2snabb'
test case a little bit (include more security arguments).