fluentd: log to a file instead of stdout

fluentd logs currently to stdout, which is known to produce big docker logs
in /var/lib/docker. This change makes fluentd to log to /var/log/kolla/fluentd.

Closes-Bug: #1888852
Change-Id: I8fe0e54cb764a26d26c6196cef68aadc6fd57b90