feat(*): allow custom component images

Pull image names for Deis components from etcd, defaulting to the
stock Deis component images. This enables users to supply their own
router, for instance, simply by setting an etcd key.

TESTING: first, make sure you can provision Deis just as before - nothing needs to change.
Then, get a little funky:
```console
dev $ make -C router/ full-clean
coreos $ etcdctl set /deis/router/image "carmstrong/router:latest"
dev $ make install-routers start-routers
dev $ curl -I http://172.17.8.100/chris-is-cool
HTTP/1.1 200 OK
Server: nginx/1.6.0
Date: Thu, 05 Jun 2014 00:19:45 GMT
Content-Type: text/plain
Content-Length: 0
Connection: keep-alive
```

Bonus points for testing an image on a private registry (haven't yet tested).

closes 1023