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.

Marcin
If Postfix isn't on the system at all, the plugin should not return any data. This is the pattern that other ohai plugins tend to follow.