Apply lowercase normalizer to certain keyword fields in template

By applying a [lowercase normalizer](https://www.elastic.co/guide/en/elasticsearch/reference/current/normalizer.html) to keyword fields the value will be lowercased in indexing time as well as at search-time. This helps if, for example, an application writes `network.type` as `IPv4` or if a user queries for `IPV4`.

I think it makes sense to apply this to fields where case-sensitivity doesn't matter.