Add fluent-logger to images

There is proposed change in kolla-ansible below
which is changing the way how central-logging
works and adding native fluent logging.

Because of this, it's needed to add python
fluent library to openstack images.

Proposed Change:
 - https://review.opendev.org/#/c/755775/

Change-Id: Iedc782a3317c93a2af0da4696fd71c05e0e4a55e

docker/openstack-base/Dockerfile.j2

@@ -89,6 +89,7 @@ LABEL maintainer="{{ maintainer }}" name="{{ image_name }}" build-date="{{ build
         'elasticsearch',
         'eventlet',
         'fasteners',
+        'fluent-logger',
         'funcsigs',
         'futurist',
         'gnocchiclient',