Setting kafka config through env vars doesn't work

Repro steps:

Run:
`KAPACITOR_KAFKA_0_ID="kafka-cluster" KAPACITOR_KAFKA_0_BROKERS_0="192.168.99.100:31092" KAPACITOR_KAFKA_0_ENABLED=true kapacitord`

Kapacitor logs this and exits:

```
run: create server: invalid configuration: kafka: no brokers specified, must provide at least one broker URL. To generate a valid configuration file run `kapacitord config > kapacitor.generated.conf`.
```

This is useful for easily configuring kapacitor running as a docker container.

Reproduce the issue with the KAPACITOR_KAFKA_0_* environment variables and the kapacitord command. Trace how these variables are converted into the Kafka configuration, then verify that the broker value is retained and kapacitord starts without the “no brokers specified” error. Add or update coverage for this environment-based configuration if the repository has a relevant test location.

Written by the indexing model from the issue text.