Add OS to docker info

Docker-DCO-1.1-Signed-off-by: Tibor Vass <teabee89@gmail.com> (github: tiborvass)
All the majors I'm aware of have support for "os-release" now.  I think having "lsb-release" as a fallback wouldn't be a bad thing, but I would be very very wary of going any further than that (and even supporting "lsb-release" is a little questionable at this point).

If we start supporting esoteric oddities like TCL's script or whatever else, we'll end up with something as nasty as what Facter's got that has very little benefit to us except for some minor debugging output.  I'd be much more interested in adding an "os-release" file to b2d (which wouldn't be too hard to create during the build) than creating a bunch of code that's incredibly difficult to test sufficiently.