Fixes fluentd for rocky

Pin fluentd version to 1.8.x

fluentd has dropped support for ruby 2.3 and below at the end of 2019.

1) https://www.fluentd.org/blog/drop-schedule-announcement-in-2019
2) fluent/fluentd#2750

Closes-Bug: #1860701

Change-Id: I29f54dc94a6e49118ce15401017947c07ed3ca54

docker/fluentd/Dockerfile.j2

@@ -60,10 +60,13 @@ RUN chmod 755 /usr/local/bin/kolla_extend_start
 {% block fluentd_plugins_install %}
 
 {% if base_distro in ['debian', 'ubuntu'] %}
+# NOTE(osmanlicilegi): fluentd has dropped support for ruby 2.3 and below at
+# the end of 2019 so we need to pin its version to 1.8.x.
 # NOTE(mgoddard): fluent-plugin-elasticsearch requires faraday, which from
 # 0.16.0 depends on Ruby 2.3, but td-agent provides its own older version of
 # Ruby.
     {% set fluentd_plugins = [
+        'fluentd:"<1.9.0"',
         'faraday:"~>0.15.4"',
         'fluent-plugin-elasticsearch',
         'fluent-plugin-grep',

releasenotes/notes/fluentd-pin-version-for-ruby-deprecation-000c7c4ab9e407ee.yaml

@@ -0,0 +1,5 @@
+---
+fixes:
+  - |
+    Pin fluentd version to 1.8.x for rocky as fluentd has dropped
+    support for ruby 2.3 and below at the end of 2019.