Running nginx-image-server on CoreOS

## WHAT
Run nginx-image-server as systemd unit on CoreOS.
So added some files to do it.

* Vagrantfile
* user-data.erb.yml
* config.rb

See [coreos/README.md](https://github.com/wantedly/nginx-image-server/blob/b2abc5b14736799cd3c75859d4e07c8a9768205c/README.md)