Support utf8 in external file alertmanager config

This could have been achieved either by switching to stringData, or doing
`std.base64(std.encodeUTF8($._config.alertmanager.config))` as per
google/jsonnet#575

I went with the former, because it's:
1. Easier to read existing config
2. Consistent with the way jsonnet object-based config is written just above