Add juju mongod

This is part of the fix for https://bugs.launchpad.net/juju-core/+bug/1351019.  It eliminates a hard-coded path.
This is a consequence of our tendency toward enormous files and packages, so I'm not sure our code base is the best guide here. :)  It's an extreme example, but just look at mgo.go (before my refactor).  Ideally I'd split it up into multiple files in an mgo package (which I still think would be a good idea), but my goal here was to minimize the effective changes (more than just moving things around).  However, I concede the even that much is unnecessary for the specific need at hand.  I was just trying to leave the code better than I found it. :)  To be honest, at this point this patch is pretty low priority and I may just drop it entirely.