F/add syslog ng modules

Hi all!

I developed a state and execution module for syslog-ng. This is my second PR against this repo, at the first time I was asked [1] to make some improvements on my code in salt-contrib [2],[3], [4].

Now I hope it's ready for integration, could you review it? I used some classes, because the original fully procedural logic was hardly maintainable. 

I have an open issue about merging these modules: https://github.com/saltstack/salt/issues/14716.

Thanks for your help :)

[1] https://github.com/saltstack/salt/pull/14033
[2] https://github.com/saltstack/salt-contrib/pull/88
[3] https://github.com/saltstack/salt-contrib/pull/87
[4] https://github.com/saltstack/salt-contrib/pull/86