Added log_level off and log_file definition support

[oomchiller]

Hey there
I enjoying using your module, and want to commit some improvements that appeared in new versions of Fluent Bit, which is:
log_level=off
log_file=/path/to/file.log

I'm not a great coder, maybe I made some mistakes somewhere, but i tested it locally, and so far it works, so I'll be glad if you take a look and merge if it's ok

[oomchiller]

I made some fixes, like renaming loglevel to "warn" according to official fluent-bit docs, added documentation to init.pp, and rewrite log_file_config variable undef logic

Now it should be fine

[oomchiller]

If i did something wrong in terms of codestyle, pls let me know, i'll try to refactor

[deric]

LGTM, thanks!