Add possibility to define plugins in meta

Change-Id: I7035f1a5f3affc2a02732fb17e7cf56d8f718a66
diff --git a/tests/pillar/agent.sls b/tests/pillar/agent.sls
index a5afd84..e2ba794 100644
--- a/tests/pillar/agent.sls
+++ b/tests/pillar/agent.sls
@@ -1,6 +1,9 @@
 fluentd:
   agent:
     enabled: true
+    plugin:
+      fluent-plugin-elasticsearch:
+        gem: ['fluent-plugin-elasticsearch']
     config:
       label:
         monitoring: