Use DEB pkgs for Fluentd plugins

On mirror.fuel-infra.org there is a package
td-agent-additional-plugins which ships all the additional
fluentd gems (plugins and its deps).

Change-Id: I0f66c793de67e9574d38b30ee3f62d534aa0bb75
Related-Bug: PROD-17532

linux/meta/fluentd.yml

 agent:
   plugin:
     fluent-plugin-systemd:
-      gem: ['fluent-plugin-systemd']
+      deb: ['td-agent-additional-plugins']
   config:
     label:
       default_metric: