ref(user-data): switch get_image to use /deis/platform/version

The `/deis/release` keyspace doesn't work with our `deisctl` configuration convention.  This PR allows users to change platform release with:

```console
$ deisctl config platform set version=v0.13.1
```

Note `/run/deis/bin/get_image` will need to be updated manually on systems provisioned with old user-data.