[bug] Fix pathing for provider.json.

This web.HttpFileHandler(srv, "/provider.json", opts.ApiPath+"provider.json") should have been opts.ApiPath+"/provider.json". The difference is there was a missing / before provider.json. That results in vpnweb trying to open a path that doesn't exist: /etc/leap/config/vpnprovider.json.

Because this is so hard to see, its better to use filepath.Join, so these things cannot happen. So I've switched these to the standard library function that joins paths.

Thanks to ale for the help!