make nats_stream_forwarder optional

nats_stream_forwarder seems broken?  so this commit allows you to disable it by adding "properties.syslog_aggregator.nats_stream_forwarder: false" to your deployment manifest.

    root@ccc25b0b-a75a-4eb8-ba83-390c25799741:~# tail -3 /var/vcap/sys/log/monit/nats_stream_forwarder_ctl.err.log
    <internal:lib/rubygems/custom_require>:29:in `require': no such file to load -- nats/client (LoadError)
            from <internal:lib/rubygems/custom_require>:29:in `require'
            from /var/vcap/jobs/syslog_aggregator/bin/nats_stream_forwarder.rb:4:in `<main>'