Unify NFV and app configuration

[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`.

Moved `equal` from `core.app` to `core.lib` because it really belonged there in the first place and I needed it for the `store_conf`/`load_conf` unit tests in `core.lib`.