Update config.pp

This ensures the config is written after the directory exists:

```
Error: /Stage[main]/Logstashforwarder::Config/Logstashforwarder_config[lsf-config]/ensure: change from absent to present failed: Could not set 'present' on ensure: No such file or directory - /etc/logstashforwarder/config.json at 131:/tmp/puppet-server/puppet/modules/logstashforwarder/manifests/config.pp
```