Add the ability to set a custom indexer template

I added the parameter $logstash::config::indexer_template so I could specify a custom template for the indexer.  If the parameter is not set then it defaults back to the old behavior.

Style note: I used a single parameter instead of indexer_template_input, indexer_template_filter, and indexer_template_output because it seemed like a cleaner and more flexible implementation.  If I want to use a combination of templates from puppet-logstash and private repo, I can use partial templates like below.
```erb
<%= scope.function_template(["logstash/indexer-input-redis.conf.erb"]) %>
<%= scope.function_template(["stash/empty-filter.conf.erb"]) %>
<%= scope.function_template(["logstash/indexer-output.conf.erb"]) %>
```