[unified conf] Replace all uses of `dofile' and `loadstring' with the
central `core.lib.load_conf' and `core.lib.load_string'.

Provide central `lib.load_conf' and `lib.store_conf' to read and write
config files and use `store_conf' to generate zone configurations in
`designs/neutron/neutron2snabb'.

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