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:

/deis/{component}/image

Also, the Deis release version is now stored in /etc/deis-release.
This is by default appended when getting the image names for Deis
components, but can be overridden if set in etcd at /deis/release.

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
```

You can also override the release:
```console
dev $ make uninstall
coreos $ etcdctl set /deis/release v0.9.1
dev $ make run
```

And test a private registry:
```console
dev $ make -C router/ full-clean
coreos $ etcdctl set /deis/router/image phife.atribecalledchris.com:5000/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
```

closes #1023