added gem plugin installation to acceptance test

examples/test.pp

@@ -0,0 +1,10 @@
+# This file is use for the beaker tests.
+
+# basic installation
+include '::fluentd'
+
+# install a gem plugin
+::fluentd::plugin { 'fluent-plugin-elasticsearch':
+  type    => 'gem',
+  require => Class['::fluentd']
+}