Support `build-spec` for other chains than Polkadot

The problem when building a chain specification is that you require the
native runtime to parse the json file (assuming the chain spec is not
raw yet). Before this pr we could only overwrite the native runtime when
running the node using `force_*`. This pr now adds support to load the
native runtime when the filename starts with the name of the chain. So,
when usng `build-spec --chain rococo-something-else.jon` it will use the
rococo native runtime to load the chain spec.