Support configuration of custom fluentd outputs

In some scenarios it may be useful to configure custom fluentd outputs
to forward logs to a logging service other than elasticsearch.

This change supports configuration of fluentd outputs by placing
output configuration files in /etc/kolla/config/fluentd/output/*.conf.

Change-Id: I3c0b271d88dbb307ba3a23546e29c72e8baeca55
Implements: blueprint fluentd-custom-outputs