Minor: Error message wording for invalid config w/ Configuration Management enabled

*Original comment by @jakelandis:*

When starting X-Pack with configuration management and a path.config in the logstash.yml the followin error is displayed.

```
ERROR: You cannot use -f since Elasticsearch is configured as the config store.
```

This error message should be changed to (or the like) :

```
ERROR: Invalid configuration: path.config (or -f) can not be set when Configuration Management is enabled.
```