Adding postfix plugin for ohai-solo

I wrote a little plugin for ohai-solo to discover current postfix configuration.
I decided to make it work only in case of RPM/DEB installed packages, and to look only at very few, but important config options, which can tell why mail delivery might not be working or not.

I thought about adding a method to check the size of outgoing mail queue, however I remember that running mailq on a server that has a huge queue, will take ages, before it produces the result you want, therefore I decided not to include that, to avoid affecting waldo or the target server.

Marcin
Since you're already parsing `postconf` output on line 44, I think you can remove this function altogether. Just add `config_directory` into `importantKeys`.