Plugins: Disable support for config dir

When removing and installing again the plugin all configuration files will be removed in `config/pluginname` dir.
This is bad as users may have set and added specific configuration files.

We don't have yet an upgrade command which could handle this nicely.

So we remove that code for now and we will implement an upgrade method in another PR.

Related to  #5064.
add some message to the assertions here so it makes more sense than `expected true/false` in case it fails.

Can you add an `ElasticsearchAssertions.assertFileExists` (for both `File` and `Path`) and `ElasticsearchAssertions.assertDirectoryExists` as shortcuts, I guess we can reuse that a lot