Add Vagrantfile

## WHY

* enable to debug this repo easily

(https://github.com/wantedly/infrastructure/issues/57)

## WHAT

* nginx-image-server on CoreOS VM via Vagrant
* Not using fleet and etcd in CoreOS
* Using "vagrant shared folder"
* Rename `script/cibuild` to `script/test`

@awakia Please try and review this instruction:

```bash
# on your local machine
$ git clone https://github.com/wantedly/nginx-image-server.git && cd nginx-image-server
$ vagrant up
$ vi Dockerfile

# login to VM and test it
$ vagrant ssh
@core-01 $ cd share
@core-01 $ docker build -t target .
@core-01 $ script/test
```
> config.rbで作ってるんだね
> 
> 失礼しました

いえ、細かい部分まで聞いてもらって理解深めた方がよいので全然ありがたいです！