Add fileset for system module

[widhalmt]

fixes #54

Beginning with release version 8 you need to give a fileset for "system" module of filebeat. Version 7 just used defaults.

Since we are thinking about replacing modules by elastic agents and filebeat only without modules in special cases, I add a quick fix and flag it as experimental. Let's see how it works.

[widhalmt]

My initial diagnosis was wrong: https://discuss.elastic.co/t/filebeat-setup-exiting-module-system-is-configured-but-has-no-enabled-filesets/311584 . We just need to change the way we setup modules. Please note that #77 could make using modules in filebeat completely obsolete. Therefore I didn't put much effort in clean code for enabling modules. If we want to continue to support, we should find a better way than just running the same code over and over again.

[widhalmt]

Default configuration with added modules will connect Filebeat to Logstash and then run filebeat setup. For all that I know, this won't work. (Even when it did in a local test). So either don't allow that combination or skip setup when talking to Logstash.

Or, the most sophisticated solution: Make sure, Filebeat talks to Elasticsearch when doing setup and then connect to Logstash. This could help with working with differently configured beats. But it would need more thinking than what I have time for now.

[danopt]

Looks good to me 👍