Load Directory Service can't load new handlers

Kapacitor 1.5.2 will refuse to start unless all handlers from the `handlers/` directory have already been previously defined with `define-topic-handler`

```
Jun 13 15:37:25 xxx kapacitord: run: open server: failed to reload tasks/templates/handlers: failed to load file /opt/tick/kapacitor/handlers/areca.yaml: unknown handler: "areca"
```

Start by reproducing startup with a new handler file in the handlers/ directory, then trace the directory-loading path that reports unknown handler. Check existing tests around handler loading if present. Done means Kapacitor starts and loads a newly added handler without requiring a prior define-topic-handler.

Written by the indexing model from the issue text.