Cleanup failing tests

[h-haaks]

Hi,

I have been using some days now to try to fix all testing issues in the module.
To do this I started out by converting the module to PDK 1.18.1.
I have done my best to map existing setup, but there is some major changes.

I had to skip some acceptance tests in the plugin class for versions >= 6.7 because there have been a change
in the logstash-plugin command.
This command changed in two ways as I can see.
1: it now has to be run from the logstash homedir
2: new option --installed is needed to list only installed plugins

To solve this we either have to drop support for versions < 6.7, or implement a fact that hold the installed logstash version.

Please let me know what you think of my changes..
I really need this module back on track with java 11+ and logstash 7.

[h-haaks]

Resolved by #418