Pin td-agent to 4.0.* to fix missing logs

The recent inclusion of Fluentd 1.12.x in Centos/Ubuntu packages
causes a significant number of logs to fail to appear in the
Elasticsearch or Monasca output plugins. The logs appear to
sit in a local buffer, and are only flushed when Fluentd is
restarted. The same issue persists in the most recent release
(1.13.0), so for now we pin the td-agent package to 4.0.* which
should restrict Fluentd to the 1.11.x release where we don't
see this issue.

Backport to Victoria.

Change-Id: Iefcdd3100b7e3c5320bc5f1286a18251bdeab885
Closes-Bug: #1930867

docker/base/apt_preferences.ubuntu

@@ -5,3 +5,8 @@ Pin-Priority: 1000
 Package: erlang*
 Pin: version 1:23.*
 Pin-Priority: 1000
+
+# FIXME(mgoddard): Pinning to 4.0.* to avoid bug 1930867.
+Package: td-agent*
+Pin: version 4.0.*
+Pin-Priority: 1000

docker/fluentd/Dockerfile.j2

@@ -27,8 +27,9 @@ LABEL fluentd_version="0.14" fluentd_binary="td-agent"
     ] %}
 
     {% if base_arch in ['aarch64', 'x86_64'] %}
+        # FIXME(mgoddard): Pinning to 4.0.* to avoid bug 1930867.
         {% set fluentd_packages = fluentd_packages + [
-            'td-agent'
+            'td-agent-4.0.*'
         ] %}
     {% else %}
         {% set fluentd_packages = fluentd_packages + [

releasenotes/notes/bug-1930867.yaml

@@ -0,0 +1,6 @@
+---
+fixes:
+  - |
+    Fixes an issue with logs going missing in the Fluentd pipeline
+    by pinning td-agent to 4.0.*.
+    `LP#1930867 <https://launchpad.net/bugs/1930867>`__