change naming strategy as discussed on irc

This avoids the ability for plugins to pollute `riemann.config`'s namespace, all plugins will be required using
the plugin name. For instance a plugin with a metadata file containing:

```clojure
{:plugin "foo" :require riemann.plugin.foo}
```

Will be available as `foo` within `riemann.config`. This keeps easy discoverability of plugins while retaining an enforced clean namespace.