Ubuntu 14.04

Two reasons:

* Backwards-compatiblity.  De facto, no instance files will currently have this.
* The OS version isn't static.  While I don't recommend it ever, people could do an in-place distribution upgrade.  At that point, the instance file would be out-of-date.

The efficiency aspect does concern me.  I tried to limit it to a single extra hostname call.  But I was also trying to be conservative.  Alternative solutions are very much welcomed.