Shared config should not failed when no sharedconfig is present.

[joek]

In line 25-27 of add_config.pp the configuration is failing with "logstash::shipper::sharedconfig is required for logstash::shipper:sharedconfig::add_config".

I can't see, why this is a case, because if no logstash is configured the configuration is not needed and should just skip. If this raises an error all plans usings this will fail until you configured logstash. (in our case production like environments only)