Less work for broadcast commands to take on.
Also fixes a bug where rollback on hosts without a running container
would stop the container they had just started.
If we don't supply a version when deploying we'll use the result of
docker image ls to decide which image to boot. But that doesn't
necessarily correspond to the one we have just built.
E.g. if you do something like:
```
mrsk deploy # deploys git sha AAAAAAAAAAAAAA
git commit --amend # update the commit message
mrsk deploy # deploys git sha BBBBBBBBBBBBBB
```
In this case running `docker image ls` will give you the same image
twice (because the contents are identical) with tags for both SHAs but
the image we have just built will not be returned first. Maybe the order
is random, but it always seems to come second as far as I have seen.
i.e you'll get something like:
```
REPOSITORY TAG IMAGE ID CREATED SIZE
foo/bar AAAAAAAAAAAAAA 6272349a9619 31 minutes ago 791MB
foo/bar BBBBBBBBBBBBBB 6272349a9619 31 minutes ago 791MB
```
Since we already know what version we want to deploy from the config,
let's just pass that through.
