Add a util/exec interface for testing execs.

Peeled off from ip-per-service
Yeah, os/exec is nice to use but a testability disaster :(

I think previously we've been testing things at a higher level (like the "DockerPuller" thing) instead of writing this generic exec testability wrapper. After seeing such a thing written out, I'm not convinced that's the wrong approach. But I'm not sure what you're going to do with this.